macroalgal bloom dynamics in a highly california estuary

Estuaries Vol. 24, No. 4, p. 623-635 August 2001

Macroalgal Bloom Dynamics in a Highly

California Estuary

Eutrophic Southern

I~RISTA t~AMER*, I{&RLEEN A. I~O'~%E, a n d PEGGY FONG

Departmer~t of O,~ganismic Biology, Ecology and Evoh~tion, University of Califmnia, Los Angeles, Los Angeles, CaliJbr,zia 90095 i606

ABSTRACT: A 10-too long moni to r ing s tudy was carr ied out in U p p e r Newpor t Bay es tuary (UNB), O r a a g e County, Cafifornia, to quant i fy the macroalgal communi ty of a s o u t h e r n Cal ifornia estuary. Quar te r ly sampl ing began December 1996 at 8 s ta t ions a long the main channel and tidal creeks ranging f rom the head to the lower end of UNB. At each station, two strata (one at high and one at low elevation) were surveyed. Macroalgal species abundance (% cover an d biomass) and algM tissue ni t rogen (N) and p h o s p h o r u s (P) were measured . The algal c o m m u n i t y changed f rom sparse macroalgal cover dur ing winter 1996 to larger pa tches domina ted by Euteromorpha i~ttesti~a.lis in sp r ing 1997. The communi ty was character ized by a thick cover of macroalgae compr i sed of E. i~ttesti~a.lis and Ulva exI)ausa in s u m m e r 1997 and U. expausa and Ceraorium spp . in fall 1997. UNB re tu rned to sparse macroalgal cover by sp r ing 1998. In s u m m e r and fall 1997~ biomaxs of E. i~ttesti~alis and C,,raoEum r eached over 1~000 g wet wt m a each~ and U. exI)ausa biomaxs exceeded 700 g wet wt m a. T i s sue N was high in E. i~ttesti~alis and U. expausa col lected f r om UNB (~3% dry wt) and higher in Ceramium (~3 .5% dry wt). Tissue P in all th ree algae ranged f rom 0.24-0.28% dry wt. T i s sue N:P (molar) ratios in E. i~ttesti~talis and U. e~pmtsa r anged f rom 16.4 to 30.0 and in Ceramium f rom 21.8 to 40.1. A field e x p e r i m e n t was conduc t ed in which E. irttestin~lis was u sed a.s a bioa.ssay of N and P availability. Algal t issue was cu l tured unde r known condi t ions and samples were deployed th roughou t the es tuary and lef t for 24 h. T i s sue N of algae f rom these bags showed a nominal increase in N with proximi ty to file p r imary nu t r i en t input to the system~ San Diego Creek (p = 0.0251; r~ = 0.200). O u r data indicate that UNB is already a highly eut rophic ~stuary~ but macroalgal b looms in UNB may increase if more N is a d d e d to the system.

In t roduct ion

Eutrophicat ion of coastal systems is increasing worldwide (e.g., Lowthion et al. 1985; Valiela et al. 1992; de Jonge 1995; Duarte 1995; McComb and Lukatelich 1995; Nixon 1995; Paerl 1997). Aug- men ted supplies of nutrients, specifically ni t rogen (N) and phosphorus (P), to coastal systems often have critical effects on coastal ecosystems by increasing both pr imary productivity (Hatcher and Larkum 1983; Oviatt et al. 1986; Lapoin te 1989; Lapointe et al. 1992; McGlathery et al. 1992; Duar- te 1995; Nixon 1995) and biomass accumulat ion of macroalgae (Delgado and Lapointe 1994; Fong et al. 199ga,b). In tempera te zones, mar ine systems are typically limited by N, though secondary limitation by P can occur (Howarth 1988; Wheeler and Bj6rnsS, ter 1992; Duarte 1995; Nixon 1995; Taylor et al. 1995).

The association between increased an thropogen- ic nut r ien t loads and nuisance blooms of macroal-

* Cor re spond ing author: p r e s e n t address: S ou t he rn Califor- n ia Coastal Water Research Project, 7171 Fenwick Lane, West- minster , California 92683; tele: 714/'372-9237; e-mail: kristak@ sccwa-p, org.

gae has been documen ted in many locations th roughou t the world (Rudnicki 1986; Raffaelli et al. 1989; Valiela et al. 1999; Geertz-Hansen et al. 1993; Peckol et al. 1994; Marcomini et al. 1995; Peckol and Rivers 1995a, b; Hernf indez et al. 1997; Hauxwell et al. 1998). These blooms are often composed of opportunist ic macroalgae in the gen- era Bntero,mc~rpha, Ulva, and Orad[a,ria. Bntero,mc~rpha and Ulva spp. have high nu t r ien t uptake rates (Ro- senberg and Ramus 1984; Fulita 1985; Duarte 1995) as do species of red algae such as Cera'miu,~z ,rMn-t~,m (Pedersen and Borum 1997) and �9 (Peckol et al. 1994). High uptake rates, combined w-ith the ability to store nutr ients (Fujita 1985; Duke et al. 1989; Fong et al. 1994; Aisha et al. 1995), may allow estuarine macroalgae to prolif- erate in systems with episodic nut r ien t fluxes. As a result, coastal estuaries subject to pulses of high n u t r i e n t s are o f t en c h a r a c t e r i z e d by seasonal blooms of macroalgae (Lowthion et al. 1985; Souls- by et al. 1985; Sfriso et al. 1987; Raffaelli et al. 1989; Valiela et al. 1992; Peckol et al. 1994; Duarte 1995; Nixon 1995; Hern~ndez et al. 1997; Sfriso and Marcomini 1997).

While macroalgae are a natural c o m p o n e n t of

�9 2001 Estuarine Research Federation 623

6 2 4 K. Kamer et al.

estuaries, excessively large b looms can have nega- tive ecosystem-wide effects. They can cause anoxic condit ions by periodically blanket ing sediments (Sfriso et al. 1987; Young et al. 1998) thereby caus- ing shifts in sed iment intZaunal communi t ies (Raf- tZaelli et al. 1991; Ahern et al. 1995). Cellular res- pirat ion by algal mats ei ther at n ight or dur ing the day when light is below the compensa t ion point in the bo t tom layers of the mats can deplete oxygen in the water co lumn (Sfriso et al. 1987; Valiela et al. 1992). This can result in fish and inver tebra te mortality, which may ult imately affect birds and other fauna in the food web (Raffaelli et al. 1989).

Macroalgae can often remove pulsed nu t r i en t inputs f rom the water co lumn before the}, can be detected by tradit ional water sampling me thods (Fong et al. 1998). In pulsed systems, water co lumn nutr ients rarely corre la te with p r imary p r o d u c e r a b u n d a n c e or productivity, a l though macroalgal tissue N and P values are known to reflect the am- bient nu t r i en t condi t ions that an alga recently expe r ienced (Bi6rns~ter and Wheeler 1990). Algal tissue N:P ratios can also be used to infer l imitat ion by ei ther N or P (Wheeler and Bj6rnsS, ter 1992). Tissue N and P conten t can thus be used as a bioassay to estimate relative differences in nu t r i en t levels in different areas of an estuary (Lyngby 1990; Lyngby and Mor tensen 1994; Hor rocks et al. 1995; Fong et al. 1998).

Cal ifornia 's wetlands are i m p o r t a n t habi ta t for a m u l t i t u d e of o r g a n i s m s as r e fuges , n u r s e r i e s , b reed ing grounds , and foraging areas (Zedler 1996). The}, have also been th rea tened by h u m a n deve lopmen t in the last century. Much wetland acreage has been lost and su r round ing urbaniza- tion heavily affects that which does r ema in (Wil- l iams and Z e d l e r 1992). U p p e r N e w p o r t Bay (UNB) is one of the largest r emain ing estuaries in sou the rn California. It is s u r r o u n d e d by urbanized areas and is subject to both po in t and n o n p o i n t sources of N and P pol lut ion (Cal ifornia Regional Water Quality Control Board 1997). Nut r ien t inputs and their effects have not been well s tudied in sou the rn California (Williams and Zedler 1992; but see Peters et al. 1985; Fong 1986; Rudnicki 1986) and there are no publ ished repor t s of algal communi ty pe rcen t cover, biomass, or tissue nutrient con ten t in sou the rn Cal ifornia (for other geo- graphic locales see Lowthion et al. 1985; Pregnall and Rudy 1985; Sfriso et al. 1987; Peckol and Riv- ers 1995a; Herngmdez 1997). T h e olzlective of this research was to characterize b looms of macroa lgae in a sou the rn Cal ifornia estuary in te rms of sea- sonality, magni tude , and tissue nu t r i en t content .

Materials and M e t h o d s FIELD SURVEYS

We conduc ted a l~InO stud), of UNB to determine macroalgal c o m m u n i ~ ~ dynamics. Quar ter ly

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Fig. 1. Location of Upper Newport Bay estuary in Orange County, California, with 8 permanent transects (1-8) and sites of the E~teromorpha i.mestir~alis bioassay exper iment (A-E) marked.

moni to r ing of the algal communi ty began in De- cember 1996, and con t inued th rough March 1998 at 8 p e r m a n e n t l y established transects in UNB (Fig. 1). Transects were chosen a long a g rad ien t f rom the head of the bay, where the p r imary fresh- water input is, to the lower end of UNB, where the highly developed and modif ied Lower Bay begins. Four transects were established a long the main channel (1, 4, 5, 8), two each on the east and west banks. Ano the r four transects were located in tidal creeks (2, S, 6, 7), two of which were off the main channel to the east side, and two to the west side. We expected to see grea ter effects of nu t r i en t en- r ichment , such as h igher pe rcen t cover and biomass, and grea ter tissue nu t r ien t con ten t nea r the m o u t h of San Diego Creek, the main nu t r i en t source, as c o m p a r e d to the m o r e distant sampl ing sites at the lower end of the estuary.

At each transect we measured a long two S0-m strata: the u p p e r s t ratum was 0.5 m toward the water f rom the vascular vegeta t ion line. T h e lower s t ratum was parallel to the u p p e r s t ratum and 1.5 m toward the water f rom the vascular vegeta t ion line. In the tidal creeks, which had s teeper banks than the ma in channel , the lower strata ran a long the bo t tom of the creek. We sampled a long the edges of the ma in channel and in the tidal creeks to character ize the macroa lga l community" in bo th types of environments .

To estimate a b u n d a n c e of macroa lgae we used mea~surements of bo th pe rcen t cover and algal biomass because bo th types of m e a s u r e m e n t s can ex- hibit high var iance when algae are distributed ir- regularly (Sfriso et al. 1987). A 0.25-m e quadra t s t rung with fishing line with 36 intercepts was placed on the ben thos at ten r andomly chosen

points along each stratum and percent cover of macroalgal species was recorded. As our second estimate of abundance , we measured algal biomass in seasons where it was present in sufficient quan- tities to allow collection. The macroalgal community of UNB was dominated by Entero,mo~phca intes tit~cdis-, Ulvca expat~sa, and fi lamentous red Cerca,mi,~m spp. These algae often become embedded in the sediments on which they grow and can be difficult to separate from mud. We randomly selected five of the ten quadrats where percent cover counts were conduc ted and placed a plastic cylinder 26.5 cm in diameter on the benthos. To standardize our collection of algal biomass, we collected all the algae inside for 90 s. Algae were kept in a cooler in the field and re turned to the laboratory within 6 h. There they were sorted to remove debris, mud, and animals, rinsed briefly w-ith distilled water to remove salts, and separated to species. Algae were placed in a nylon mesh bag, spun in a salad spin- ner for I minute and wet weighed, and then dried at 60~ to a constant weight. Algal tissue samples were g round with mortar and pestle and sent to the DANR Analytical Laboratory at University of California at Davis for tissue N and P analyses. To- tal N in algal tissue was analyzed by Nitrogen Gas Analyzer using induct ion furnace and thermal con- ductivity. Total P in algal tissue was quantitatively de termined by atomic emission spectroscopy fol- lowing microwave acid digestion of sample. Tissue N and P are repor ted as percent dr}, wt and molar N:P ratios were calculated fi-om these data.

Three-way analysis of variance (ANOVA factors: season, transect, stratum) was used to de termine differences in algal percent cover and algal species biomass for E. intestina~is, U. ea~ansa, and Cera,miu~ spp. To determine if data complied with ANOVA assumption of homogene i ty of variance, residuals versus fitted Y values were plotted. Unequal variances in the percent cover data were corrected by t ransforming the data with arcsin square root cal- culations. Unequal variances in the algal biomass data were corrected by taking the log (x + 1) of the data. Means reported th roughou t the text were generated from unt ransformed data. Due to the patchy nature of the algae, while it was possible to collect biomass, some samples were too small for analysis of either tissue N or R If there was not enough tissue to conduct both N and P analyses, we chose to prioritize N over R In several cases our n was <S and our sample sizes were not always equal for N and P. Due to low and variable sample sizes, we did not conduct statistical analysis of algal tissue N and P content from individual strata and transects in different seasons. To compare tissue nutr ient content of the different algae, we pooled data from each species across strata, transects, and

Macroalgal Blooms in a Eutrophic Estuary 625

seasons. One-way analysis of variance (ANOVA factor: algal species) was used to determine differences in algal tissue % N, % R and N:P ratio. Follow- ing significant one factor ANOVA, Fisher's Pro- tected Least Significant Difference test (PLSD) was used to identify differences a m o n g means.

NUTRIENT BIOASSAY

To quantify relative levels of dissolved inorganic N and P availabilit 7 in UNB along the hypothesized nutr ient gradient, we used E. ir~testir~cdis as a bioassay organism. For the bioassay technique to be successful, the algae need to be exposed to the envi ronment long enough that they can accumu- late nutrients in their tissues, but not so long that tissues become saturated and distinction between sites is lost. Because we did not know a priori the appropria te length of time required for the bioassay in LrNB, we ran it for 24 and 48 h.

Entero,mo~pha intestinalis was collected fi-om Mugu Lagoon, Ventura Count3,, California, on July 7, 1998. Algae were kept outdoors at University of California, Los Angeles, in shallow pans filled with seawater low in nutrients relative to estuarine water (22.81 -+ 0.46 IJAzI NO s, 2.10 _+ 0.09 p~M PO4, m e a n + SE) with constant aeration in order to reduce variability in initial tissue N content (Fong e ta l . 1998). The pans were kept at 25~ and covered with fiberglass window screening for 7 d. Salinity was moni to red daily and distilled water was added to compensate for evaporation. The day before the exper iment began, 5 g (_+0.1 g) sub-samples orE. ir~testir~[is were placed in mesh bags made out of nylon window screening and re turned to low nutrient seawater overnight. Initial subsamples were rinsed briefly with distilled water, dried, and analyzed for tissue N and P as described above. Initial tissue N of the algae that were placed in the field was 1.605 _+ 0.063% drywt (mean _+ SE) and initial tissue P was 0.206 + 0.005% dry wt. The mean molar N:P ratio of initial samples was 17.8 + 0.5.

On July 15, 1998, 10 bags containing algae were deployed at each of five sites in LrNB along the estuarine gradient (A-E; Fig. 1). Bags were attached to bamboo stakes with polypropylene rope and the stakes were placed at the water's edge at mid-tide. Bags were placed at similar elevations to assure that the}, were exposed to water for equal amounts of time during tidal cycles. A small float was attached to each bag to suspend it in the water column. Five bags from each site were retrieved after 24 h and the other five after 48 h. U p o n re- trieval, the bags were kept in a cooler during trans- port back to the laboratory where the algae were removed, rinsed briefly in distilled water to remove salts, dried and analyzed for tissue N and P as described previously. Simple linear regression was

626 K. Kamer et al.

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Fig. 2. S e a s o n a l a b u n d a n c e ofEnteromo~pha intestinalis ( top) , Ulva expansa ( m i d d l e ) , a n d Cera~ni~n spp. ( b o t t o m ) in U p p e r N e w p o r t Bay in W i n t e r 1996, S p r i n g 1997, Smaamer 1997, Fall 1997, a n d Spidng 1998 (bars a re -+1 SE). * i n d i c a t e s t r a t a w h e r e da ta were n o t co l lec ted .

used for each time period to determine if there was a significant relationship between distance f iom the nutr ient source and tissue N values, tissue P values, or tissue N:P ratios.

R e s u l t s

FIELD SURVEYS---PERCENT C O V E R

Percent cover of E. intestinalis showed a strong seasonal pat tern over the durat ion of the study (Fig. 2). Small anaounts were always present in at least a few locations th roughou t the 16-too study.

There were significant effects of season, transect, and stratum on percent cover (Table 1). During winter 1996, E. intestinalis comprised 30% of the cover at one stratum but otherwise was absent or present only in very low abundance (<5%). While abundance increased in spring 1997, with cover up to 81%, it remained spatially patchy. E. i~testinalis was most abundan t during summer 1997, when it ranged from 6-98% cover. Abundance of E. intes- tinaSs decreased in fall 1997. The maximum percent cover sampled was 46% but most of the strata

Macroalgal Blooms in a Eutrophic Estuary 6 2 7

TABLE 1. Sumnaary of three way ANOVA of the effects of season, stra~ma, transect , and all in te rac t ions on pe r cen t cover of Entero- morpha intestinalis, Ulva expa.nsa, and Ceg'a.r~aiu.m spp. in Uppe r Newpor t Bay (analysis p e r f o r m e d on data t rans formed by arcsm squm-e root).

>dgal Group Source df MS F p

E.i.ntesti.nalis Season 4 7.251 216.162 0.0001 Transect 7 3.003 89.529 0.0001 Stratum 1 1.762 52.528 0.0001 Season X t ransect 27 0.453 13.518 0.0001 Season • s t ra tum 4 0.447 13.340 0.0001 Transect X sh 'atum 7 0.472 14.068 0.0001 Season • t ransect • sn 'amm 24 0.686 20.448 0.0001 E::-o:- 675 0.084

gZ mj)ansa Season 4 27.377 938.200 0.0001 Transect 7 2.356 80.755 0.0001 Stratum 1 0.232 7.949 0.0050 Season X t ransect 27 1.214 41.620 0.0001 Season )< sn'aman 4 0.087 2.982 0.0186 Transect X sh 'amm 7 0.629 21.539 0.0001 Season )< t ransect )< sU'amm 24 0.604 20.685 0.0001 Error 675 0.029

C'era.mi~.m spp. Season 4 5.613 394.083 0.0001 Transect 7 2.330 165.604 0.0001 Stratum 1 3.861 271.093 0.0001 Season X t ransect 27 1.006 70.609 0.0001 Season )< sU'aman 4 1.457 102.286 0.0001 Transect • s t ra tum 7 0.770 54.094 0.0001 Season )< t ransect )< s t ra tum 24 0.497 34.887 0.0001 Error 675 0.014

had < 8 % cover, In sp r ing 1998, E. intestineSs cover r a n g e d f rom 1-24% a n d was p r e s e n t at only 4 of the strata,

T h e r e were s ign i f ican t i n t e r ac t i ons be twe e n all S factors of the ANOVA for p e r c e n t cover of E. intes tineSs (Table 1). T h e i n t e r a c t i o n be tween season a n d t ransec t was caused by cons is ten t ly low cover of E. intestinelis at t ransec t 1 (at the lower e nd of UNB) regardless of season. T h e r e was no seasona l effect on E. intestinaSs cover at t ransec t 1 whereas all o the r t ransects showed seasona l peaks of E. intestiness. Across seasons, E. intestineSs p e r c e n t cover was h i g h e r in u p p e r strata as c o m p a r e d to lower, except d u r i n g spr ing 1997 w h e n the re was no dif- f e r ence b e t w e e n u p p e r a n d lower strata. Th i s caused an i n t e r a c t i o n be tween season a n d s t ra tum. T h e r e was also a s ign i f ican t i n t e r a c t i o n b e t w e e n t ransec t and s t r a t um because the re was no consis- t en t p a t t e r n of g rea te r cover in e i the r s trata a m o n g transects .

P e r c e n t cover of U. expansa showed a s t rong seasona l p a t t e r n over the 16-mo s a m p l i n g pe r iod (Fig. 2). It was s ignif icant ly affected by season, t ransect , a n d s t r a tum (Table 1). U. ex~ansa was p r e s e n t in very low a b u n d a n c e d u r i n g win te r 1996 a n d sp r ing 1997 ( < 1 % ) , a n d it was a b s e n t f rom ou r t ransects in sp r ing 1998. U. ex~ansa was mos t a b u n d a n t during s u m m e r a n d fall 1997. Its cover r a n g e d f rom 2 - 8 2 % in s u m m e r a n d f rom 4 - 1 0 0 % in fall.

T h e r e were s ign i f ican t i n t e r ac t i ons be twe e n all g factors of the ANOVA for p e r c e n t cover of U. ex>

pense (Table 1), Whi le all t ransects showed seasonal c ha nge s in U. expansa a b u n d a n c e , the changes were n o t cons i s t en t a m o n g transects , caus ing an i n t e r a c t i o n be t w e e n season a n d t ransect , In te rac- t ion be t w e e n season a n d s t r a tum o c c u r r e d because in one season U. expense had g rea te r p e r c e n t cover in the u p p e r s t ra ta as c o m p a r e d to the lower strata, In all o the r seasons the re was n o d i f fe rence between strata. T h e r e was also a s ign i f ican t in terac- t ion be t w e e n t ransec t a n d s t r a tum; a m o n g transects, the re was no cons i s t en t p a t t e r n with r ega rd to strata.

Cera,miu~n spp. also exh ib i t ed a seasona l p a t t e r n of a b u n d a n c e (Fig. 2) with a peak d u r i n g fall 1997. Its p e r c e n t cover wa~s s ignif icant ly affected by season, t ransect , a n d s t r a tum (Table 1). It was f o u n d at only one s t r a tum in w-inter 1996 w h e n it con> pr ised 5% of the cover. In s u m m e r 1997, Cere'miu~z cover r a n g e d fi-om 1-80% at t ransects where it oc- c u r r e d . D u r i n g fal l 1997, cove r of Ce,ra,miu,m r e a c he d 100% at two t ransects a n d wa~s patchi ly dis- t r i bu t ed a m o n g transects . It was a b s e n t f rom UNB in the sp r ing seasons. "vghen C~ramiu,m was p resen t , it was m o r e a b u n d a n t in the lower strata t h a n in the u p p e r strata.

T h e r e were s ign i f i can t i n t e r a c t i o n s be t ween all th ree factors of the ANOVA. Cera,miu,,m spp. was neve r f o u n d at t ransects g a n d 4, caus ing a n inter- ac t ion be t w e e n season a nd transect , Because (_-;e~ a,miu,m was never p r e s e n t at these t ransects , they did n o t exh ib i t the same seasonal p a t t e r n s o f a b u n -

628 K. Kamer et al.

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a summer 1997 [] Ceramium sll p,

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Fig. 3. C o m b i n e d a l g a l b i o m a s s o f Er~te.romco'pha i.ntestir~lis, U~va expar~sa, and Cerar~h~za spp. for a) smnmer 1997 and b) fall 1997 showing biomass of over 1,500 g wet wt m -s.

dance that the o the r t ransects did. Th i s caused a n i n t e r a c t i o n b e t w e e n t ransec t a n d s t ra tum. Across seasons, Ceramiu~# wa,s m o r e a b u n d a n t in the lower strata, excep t for the two sp r ing seasons w h e n it was absen t fi-om UNB. Th i s caused an i n t e r a c t i o n be tween season a n d s t ra tum.

A L G A L ]~IOMASS

While E. intestinalis was p r e s e n t in all seasons as i nd i ca t ed by p e r c e n t cover data, b iomass was gen- erally sparse in win te r and spr ing seasons. Q u a n - tities were suf f ic ien t to al low b iomass co l lec t ion only" in s u m m e r a n d fall 1997. Biomass was significant ly affected by t ransect , season, a n d s t r a tum (Table 2). In summer , E. intes6natis was col lected f rom 12 of 16 s t rata in a m o u n t s r a n g i n g f rom 3 4 - 1,141 g wet wt m 2 (Fig. Sa). At 7 of those s t rata the re was > 1 0 0 g wet wt m 2. T h e m e a n of E. in- testi,~Mis biomass in s u m m e r f rom bo th u p p e r a n d lower s t ra ta was 226 -+ 45 (SE) g wet wt m -2. M e a n E. i'ntesti'naSs biomass for fall 1997 was lower t h a n s u m m e r ( m e a n - 62 + g6 g wet wt m ~), a n d E. intestir~Mis was less a b u n d a n t t h a n U. expcmsca a n d Cera,miwm spp. in fall (Fig. 3b) . Biomass of E. it~tes ti'nafs exceeded 100 g wet wt m -~ at only 4 of 11 s t rata f rom which it was col lected.

ANOVA for b iomass of E. i'ntestinMis showed sig- n i f i can t i n t e r a c t i o n s be tween the th ree factors (Ta-

ble 2). T h e i n t e r a c t i o n be t w e e n season a n d t ransect was caused by peaks in a b u n d a n c e in s u m m e r 1997 at two t ransects a n d a peak in a b u n d a n c e in fall 1997 at a d i f fe ren t t ransec t i n d i c a t i o n that the effect of season var ied with t ransect . T h e sea~sonal effect was lack of b iomass in win te r a n d sp r ing seasons. Across seasons, b iomass was g rea te r in the u p p e r s t ra ta in fall 1997, b u t there were no differences in b iomass b e t w e e n u p p e r a n d lower s t ra ta for s u m m e r 1997, caus ing an i n t e r a c t i o n be tween season a n d s t ra tum. T h e r e was n o t a cons i s t en t pat- t e rn of g rea te r E i~testinalis biomass in e i ther the u p p e r or lower strata with regard to t ransect , re- su l t ing in s igni f icant i n t e r a c t i o n b e t w e e n t ransec t an d s t ra tum.

g iomass of U. expansa was also only col lected in s u m m e r a n d fall 1997. It was s igni f icant ly affected by season a n d t ransec t bu t n o t by s t r a tum (Table 9). I n s u m m e r , U. expar~sa biomass r a n g e d f rom 6 - 771 g wet wt m -2 a n d was col lected everywhere except t ransec t 1 (Fig. 3a). M e a n U. expa.nsc~ biomass in s u m m e r f rom b o t h u p p e r a n d lower s t ra ta was 816 + 47 g wet wt m -e m a k i n g it the mos t a b u n - d a n t alga at that t ime t h o u g h the m a x i m u m re- co rded b iomass of U. expansa was less t h a n that of E. irttestinMis. In fall 1997, b iomass r a n g e d f rom 20-798 g wet wt m -e a nd wa,s col lected f rom every t ransec t (Fig. 3b). M e a n U. expc~.nsc~ b iomass in the fall was s imilar to s u m m e r (259 + 34 g wet wt m 2).

T h e r e were s ign i f ican t i n t e r a c t i o n s b e t w e e n factors of the ANOVA for U. expansa biomass (Table 2). I n t e r a c t i o n b e t w e e n season a n d t ransec t oc- c u r r e d because th ree t ransects had peaks of U. expar~sa b iomass in s u m m e r 1997 while two o thers had peaks in fall 1997. T h e effect of season was d e p e n d e n t u p o n t ransect . T h e r e was also no con- s is tent p a t t e r n of g rea te r b iomass in e i the r the uppe r or lower strata a m o n g transects , caus ing a sig- n i f i can t i n t e r a c t i o n be t w e e n t ransec t a n d s t ra tum.

Cera'miu~ spp. b iomass was also col lected only in s u m m e r a n d fall 1997. It was s igni f icant ly affected by t ransect , season, a n d s t r a t um (Table 2). In summer, 2 -140 g wet wt m 2. was col lected fi-om 5 strata, a n d n o n e was col lected f rom the o the r 11 (Fig. Sa). T h e m e a n of Cera~ium bioma,ss in s u m m e r fi-om b o t h u p p e r a nd lower strata was 20 -+ 6 g wet wt m e m a k i n g it the least a b u n d a n t alga d u r i n g that season. In fall 1997, 5-1 ,573 g wet wt m -~ was col lected f rom 1.9 strata (Fig. 3b) . P~iomass exceeded 1,500 g wet wt m ~ at two s t rata a n d was >1 ,000 g wet wt m e at a third. M e a n b iomass was m u c h h i g h e r t han in s u m m e r (342 + 73 g wet wt m-2) , a n d Cera,mi.zcm was the mos t a b u n d a n t alga d u r i n g fall 1997.

T h e r e were s ign i f ican t i n t e r a c t i o n s be t ween the th ree factors of the ANOVA for Cera,mizcm spp. biomass (Table 2). I n t e r a c t i o n b e t w e e n season a n d


TABLE 2. S u m m a r y of th ree fac to r ANOVA of the effects of season, s t r a tum, t ransec t , and all i n t e r a c t i o n s on wet b iomass of F~nteromorpha i.ntestinalis, Ulva expansa, and Ce.ra.r~ai.u.m spp. in U p p e r N e w p o r t Bay (analysis p e r f o r m e d on da ta t r a n s f o r m e d by log(x + 1)).

#Jgal Group Source df MS F p

E.i.ntesti.nalis Season 4 4.396 73.979 0.0001 Transect 7 0.641 10.781 0.0001 Stratum 1 0.363 6.104 0.0140 Season X t ransect 27 0.381 6.406 0.0001 Season • s t ra tum 4 0.201 3.388 0.0099 Transect X sh 'atum 7 0.273 4.591 0.0001 Season • t ransect • sU'amm 24 0.237 3.986 0.0001 Error 300 0.059

U. m'pansa Season 4 13.533 262.404 0.0001 Transect 7 1.510 29.273 0.0001 Stratum 1 0.126 2.446 0.1189 Season ;~ t ransect 27 0.845 16.376 0.0001 Season )< s t ra tum 4 0.049 0.955 0.4325 Transect x sU'atum 7 0.453 8.782 0.0001 Season )< t ransect )< s t ra tum 24 0.301 5.845 0.0001 Error 300 0.052

Cera.miwm spp. Season 4 4.254 155.062 0.0001 Transect 7 1.063 88.750 0.0001 Stratum 1 2.124 77.418 0.0001 Season X t ransect 27 0.669 24.366 0.0001 Season )< su-amm 4 1.029 37.502 0.0001 Transect X sU'atum 7 0.381 13.905 0.0001 Season )< t ransect )< s t ra tum 24 0.340 12.410 0.0001 En-or 300 0.027

t r ansec t o c c u r r e d b e c a u s e o n e t r ansec t h a d g rea t - est Ceramium b i o m a s s in s u m m e r 1997 whi le 6 t ransects h a d g r e a t e s t b i o m a s s in fall 1997. I n t e r a c t i o n o c c u r r e d b e t w e e n s ea son a n d s t r a tum. W h e n col- l e c t ed , b i o m a s s was g r e a t e r in the lower s t ra ta as c o m p a r e d to t he u p p e r , b u t in t h r e e s easons no Ceramiu~n b i o m a s s was co l l ec ted . T h e r e wa~s also an i n t e r a c t i o n b e t w e e n t r ansec t a n d s t r a tum. Cera,m- iu,m b i o m a s s was g r e a t e r in the lower s t r a ta a t all t r ansec t s e x c e p t two.

T l s s u s NUTRIENTS

Tissue N (p - 0.0030, A N O V A ) a n d P (p - 0 . 0184 , A N O V A ) w e r e s i g n i f i c a n t l y d i f f e r e n t a m o n g E. intestinMis, U expansa, a n d Cerarniurn spp. co l l e c t ed f r o m LYNB in s u m m e r a n d fall 1997. Cer a,miu,m h a d the h i g h e s t t issue N at 3.477 _+ 0 .146% dr), wt ( m e a n + SE). I t v a r i e d s ign i f i can t ly f r o m b o t h U. expansa t issue N (8.086 + 0 .065% dr), wt; p - 0.0063, PLSD) a n d E. intestinaSs t issue N (2.924 + 0 .125% d ry wt; p - 0.0010, PLSD) . E. intestinalis h a d the h i g h e s t t issue P at 0.278 + 0 .009% d r y wt. It v a r i e d s ign i f i can t ly f r o m b o t h U. e,x~ansa t issue P (0.258 _+ 0 .005% d ry wt; p - 0.0447, PLSD) a n d 4>ra~nium t issue P (0.248 + 0 .009% dr), wt; p 0.0036, PLSD) . R e s u l t i n g t issue N:P (mo la r ) ra t ios were also s ign i f i can t ly d i f f e r e n t a m o n g a lga l spec ies (p = 0.0001, A N O V A ) . Cer- amium h a d the h i g h e s t m e a n N:P ra t io (32.5 _+ 1.3). I t was s ign i f i can t ly d i f f e r e n t f rom U. ax~ansa (26.3 _+ 0.3; p = 0.0001, PLSD) a n d E. intestinalis

(22.9 + 0.7; p - 0.0001, PLSD) . :7. expanse was s ign i f i can t ly d i f f e r e n t f rom E. intestinaSs (p - 0.0004, PLSD) .

T issue N o f E. intestinMis was va r i ab l e wi th in the s easons it was m e a s u r e d . M e a n t issue N r a n g e d fi-om 1 .61 -4 .22% d ry wt in s u m m e r 1997 (Table g). In g e n e r a l , t issue N o f E. intestinMis was h i g h e r a t m i d - e s t u a r y t ransec t s (Transec t s 4--7) t han at t r ansec t s a t e i t h e r e n d o f the estuary. T i s sue N o f E. intestinaSs in fall was s imi l a r to tha t m e a s u r e d in s u m m e r , t h o u g h the r a n g e it c ove re d was s m a l l e r ( 1 .87 -3 .23% d ry wt). Tissue P was h i g h e r in sun-> m e r 1997 t h a n in fall for E. intestina~is. T h e m e a n s u m m e r P va lue was 0.287 + 0.009 (SE) % d ry wt w h e r e a s in fall it was 0.282 + 0.020. M o l a r N:P ra t ios for b o t h seasons r a n g e d f r o m 16.4-28.1 . [~ expansa t issue N was h i g h e r in s u m m e r 1997 t h a n in fall (Table 4). T h e m e a n s u m m e r N va lue was 8.438 + 0.071 (SE) % d r y wt. In fall it was 2.769 _+ 0 .084% d ry wt. U. expansa t issue P was also h i g h e r in s u m m e r 1997 t h a n in fall. M e a n t issue P was 0.289 + 0 .005% d ry wt in s u m m e r a n d 0.2B5 _+ 0 .006% d ry wt in fall. Across b o t h seasons , m o l a r N:P ra t ios r a n g e d fi-om 19.8-30.0.

In c o n t r a s t to the p a t t e r n s een in U. expansa, Ceramiu~n spp. t issue N was h i g h e r in fall 1997 t han in s u m m e r (Table 5). T h e m e a n s u m m e r N va lue was 3.029 _+ 0.291 (SE) % d r y wt. In fall it was 3.631 _+ 0 .162% d r y wt. Ceramium t issue P was also h i g h e r in fall 1997 t h a n in s u m m e r . M e a n t issue P was 0.224 -+ 0 .011% d ry wt in s u m m e r a n d 0.252


TABLE 3. E, nteromorph, a i,ntestinalis tissue N with standard error and n. Transects where

and P values and tissue N:P (molar) ratios from summer and fall 1997. gleans are given no samples were collected were omitted fi'om the table.

,-qeasol: T r a l : s e c t ~qtratulTl M e a l :

T issue I',~ T i s sue P T i s sue N P

BE n M e an SE n M e an BE n

Surmner 2 Upper 2.380 Lower

8 Upper 1.610 Lower 2.688



6 Upper 3.992 Lower



Fall 3 Upper 1.866 Lower

6 Upper 3.234 Lower

7 Upper 3.035 Lower

8 Upper Lower 2.679

0.279 4 0.250 0.015 3 19.3 2.2 3

0.211 4 0.203 0.007 3 16.4 2.5 3 0.297 5 0.260 0.009 5 25.0 2.7 5 0.603 2 0.335 0.005 2 21.7 4.3 2 0 1 0.390 0 1 24.0 0 1 0.240 2 0.240 0 1 24.6 0 1 0.389 2 0.290 0 1 23.5 0 1 0.173 3 0.317 0.027 3 28.1 1.5 3

0.083 5 0.320 0.009 5 25.3 1.2 5 0.282 2 0.860 0 1 20.7 0 1 0 1 0.270 0 1 24.4 0 1 0.141 4 0.295 0.022 4 20.7 1.7 4 0.072 2 0.175 0.015 2 23.7 1.1 2

0 1 0.290 0 1 24.7 0 1

0.189 2 0.260 0.010 2 25.8 0.6 2

0 1 0.230 0 1 25.8 0 1

_+ 0.012% dry wt in fall. Highes t mean tissue N and P values of Cera,mium were measured at Transec t 7, lower s tratum, in fall 1997. Molar N:P ratios in s u m m e r and fall 1997 ranged f rom 21.8-40.1.

NUTRIENT e)I OASSAY

In the E. intestinMis bioassay exper iment , algae placed closer to the nu t r i en t source exper ienced grea ter increases in tissue N than algae fur ther fi-om the source (Fig. 4a). T h e r e was a significant positive l inear re la t ionship between proximity to the m o u t h of San Diego Creek and accumula t ion of tissue N (p - 0.0251) after 24 h. Tissue N percent change f rom initial was greater for sites at the head of the estuary, where San Diego Creek enters the estuary, than it was for transects fu r ther dowal the bay. Distance fi-om San Diego Creek only explained 20% of the variabilit3r (r 2 - 0.200). T h e significant re la t ionship detected after ,94 h disap- peared after 48 h exposure in the estuary. After 48 h pe rcen t change in m e a n tissue N at all sites ranged fi-om 54-73% and there was no consistent spatial pa t t e rn of change in tissue N a m o n g sites.

E. i~testinalis did not show consistent increases in tissue P values with increasing proximity to the nu t r i en t source after 24 h (Fig. 4b). T h e r e was no significant re la t ionship between site and tissue P pe rcen t change f rom initial (p - 0.0859, r 2 - 0.123). T h e r e was also no significant relat ionship between pe rcen t change f rom initial in tissue P and site after 48 h. Changes in tissue P were also much lower in magni tude than changes in tissue N.

T h e r e was no significant relat ionship between tissue N:P ratio and site (p 0.0747, r ~ 0.1S2; Fig. 4c). Ratios ranged f i om 1S.2-24.9 for all five sites tested.

D i s c u s s i o n

UNB is a highly eu t rophic system. T h e estuary expe r ienced large seasonal b looms of macroa lgae which are often characteristic of systems subject to large inputs of nutr ients (Rudnicki 1986; Sfi-iso et al. 1987; Raffaelli et al. 1989; Valiela et al. 1992; Geer tz -Hansen et al, 1998; Peckol et al, 1994; Mar- comini et al. 1995; Peckol and Rivers 1995a,b; Her- nSndez et al. 1997; Young et al. 1998). During sun-> m e r and fall 1997 when b looms ofE. intestindis, U expansa, and Cemrnium spp, occurred, some areas surveyed were complete ly covered w-ith macroalgae. Chrer 150 g dry wt m e of algae were collected dur ing peaks of algal a b u n d a n c e ( t ransformat ion of our biomass wet weight data to dry weight; Fong and I (amer unpubl i shed data). Research in o ther nu t r i en t rich ~ystems has yielded results within the same order of magni tude . Pregnall and Rudy (1985) r epor ted yields fi-om field collections of En tero,mm/)h,a and Ulva spp. ranging f rom 800-750 g dry wt m ~. H e r n S n d e z et al. (1997) collected 200- 875 g dry wt m -2 of U/va spp., and Lowthion et al. (1985) r epor ted lower values of 31-50 g dry wt m e ofEnteromm/)ha spp. Ano the r b loom fo rming g reen macroalga, Cladophora vagabundc~, was found in the field at densities >1 kg dry wt m -e (Peckol and Rivers 1995a). T h e red alga Gracila, r'ia tihvah, iae

Macroalgal Blooms in a Eutrophic Estuary 631

T A B L E 4. Ulva expa,usa tissue N and P values and tissue N:P (molto') ratios f r om stumbler and fal l 1997. Means are given wi th standard er ro r and n. Transects where no samples were col lected were omi t ted fl-om the table.

Tissue N T i s sue P Tissue N P

,-qe asol: T r a l : s e c t St ra tul~l M e a l : S E i: M e a l : S]~ n M e a l : S E n

Summer

Fall

2 Upper 3.306 0.115 5 0.246 0.012 5 30.0 1.5 5 Lower 5.751 0 1

3 Upper 1.991 0 1 Lower 2.940 0.685 2

4 Upper 3.527 0.076 4 0.298 0.007 4 26.3 0.5 4 Lower 3.623 0.072 4 0.308 0.008 4 26.2 1.0 4

5 Upper 3.332 0.392 3 0.267 0.022 3 27.6 2.2 3 Lower 3.048 0.594 2 0.275 0.045 2 24.4 0.8 2

6 Upper 3.977 0.132 2 0.310 0 1 27.5 0 1 Lower 3.933 0.094 2

7 Upper 3.732 0.139 5 0.306 0.005 5 27.0 0.8 5 Lower 3.852 0.144 3 0.323 0.008 8 26.4 0.8 8

8 Upper 3.349 0.036 5 0.288 0.010 5 25.9 0.9 5 Lower 3.130 0.105 5 0.288 0.010 5 24.2 1.3 5

1 Upper 2.184 0 1 0.180 0 1 26.9 0 1 Lower 2.835 0.057 4 0.245 0.005 4 25.6 0.1 4

2 Upper 2.880 0.092 4 0.222 0.009 4 28.8 1.6 4 Lower 2.926 0 1 0.220 0 1 29.5 0 1

3 Upper 1.573 0.064 3 0.153 0.009 3 22.8 0.7 3 Lower 2.143 0.123 5 0.240 0.013 5 19.8 0.6 5

4 Upper 3.056 0.132 5 0.236 0.006 5 28.7 1.2 5 Lower 2.539 0 1 0.190 0 1 29.6 0 1

5 Upper 3.169 0.360 5 0.256 0.024 5 27.3 1.9 5 Lower 2.603 0.180 5 0.230 0.009 5 25.0 0.8 5

7 Upper 3.247 0.491 2 0.275 0.035 2 26.1 0.6 2 Lower 3.354 0.180 3 0.297 0.017 3 25.1 1.6 3

8 Upper 2.840 0.249 5 0.226 0.014 5 27.7 1.1 5 Lower 2.971 0.183 5 0.246 0.012 5 26.7 0.5 5

wh ich is h igh ly a b u n d a n t in W a q u o i t Bay, Massa- chuset ts , has b e e n f o u n d at dens i t ies > 1 0 0 g dry wt m 2 (Valiela et al. 1992), which is s imi lar to the bioma~ss of the red Cera~iu~n spp. f i o m UNB.

O u r two es t imates of a b u n d a n c e , p e r c e n t cover a n d b iomass m e a s u r e m e n t s , p r o v e d to be va luab le in d i f f e r e n t seasons. W h e n m a c r o a l g a l cover was sparse , it was n o t p rac t ica l to m e a s u r e b iomass because the small , f ine p ieces o f a lgae on the sedi- m e n t su r face cou ld n o t be quan t i t a t ive ly co l lec ted . D u r i n g those t imes, cover was an effect ive tool in c h a r a c t e r i z i n g the commun i ty . M e a s u r e m e n t s o f b iomass were a g o o d tool for cha rac t e r i z ing the m a c r o a l g a l c o m m u n i t y d u r i n g b l o o m m, ents. Dur- ing s u m m e r and fall 1997, cover m e a s u r e m e n t s often r e a c h e d 100% for m a n y strata a n d did n o t allow for d i s t inc t ion b e t w e e n these strata. However , b iomass es t imates for the s a m e s t ra ta showed dif- f e r e n c e s in m a c r o a l g a l a b u n d a n c e b e t w e e n the strata.

T h e algal c o m m u n i t y of U N B showed s t rong seasonal pa t t e rns . M a c r o a l g a e were sparse in win te r a n d spr ing seasons p r o b a b l y due to l igh t a n d / o r t e m p e r a t u r e l imi ta t ion . S imi la r p a t t e r n s o f ma- c roa lga l seasona l a b u n d a n c e hm~e b e e n d o c u m e n t - ed for o t h e r areas. In C o o s Bay, O r e g o n , Entero- 'mo~J)ha a n d Ulva were a b u n d a n t J u l y - O c t o b e r

(P regna l l and Rudy 1985). L o w t h i o n et al. (1985) f o u n d that m a c r o a l g a e (pr inc ipa l ly Entero,no~pha) p e a k e d every s u m m e r for 6 yr in L a n g s t o n e Har- born-, E n g l a n d , a n d were prac t ica l ly absen t in winter. T h e L a g o o n of Venice e x h i b i t e d m a x i m u m ma- e roa lga l b iomass in late s p r i n g - s u m m e r ( M a r c o m - ini et al. 1995), a n d Sfriso et al. (1987) r e p o r t e d h i g h Ulva 'rig'ida biomass in May a n d July of o n e year. H e r n ~ n d e z et al. (1997) r e p o r t e d semi-an- nua l peaks ( J u n e and D e c e m b e r ) in a b u n d a n c e for Ugva in the P a l m o n e s River estuary, Spain . In s o u t h e r n Ca l i fo rn ia , o t h e r es tuar ies also exper i - e n c e seasona l b l o o m s of m a c r o a l g a e (Peters et al. 1985; R u d n i c k i 1986) t h o u g h the b l o o m s o c c u r in w in t e r or sp r ing seasons. Var ia t ion in the t im ing of b l o o m events a m o n g es tuar ies may be r e l a t ed to t e m p e r a t u r e or o t h e r factors that i n f l u e n c e b l o o m dynamics .

We did n o t see h i g h e r cover, b iomass , or tissue n u t r i e n t c o n t e n t o f E. intestinaSs or [L e~x~)ansa n e a r the m o u t h of San D iego C r e e k as we had expec t ed . Ins tead , a lgae were pa tch i ly d i s t r ibu ted t h r o u g h o u t the estuary. Wate r m o v e m e n t at the m o u t h o f San Diego Creek is d o w n s t r e a m , possibly p r e v e n t i n g ac- c u m u l a t i o n o f m a c r o a l g a e in the u p p e r r e a c h e s o f UNB. Entero,mmpha a n d [gva t end to f o r m la rge mats that d e t a c h f r o m the subs t ra te (Sfriso et al.

632 K. Kamer et al.

e. ~

E O

&

z

~

~

E O

& < D

O

~ 1 7 6

~

75

50

25

-25

20

15

10

5

0

-5 c)

24

21

18

15

12

a) 0

0 0 0

o ~

o o o O

b) o o

0 0 0

0 0 0

0 0 0 0

o

o

o o

o o

o 0 o o

o o o 0 o

o o

o o O o o

8 O

I I I I I

A B C D E

Lower end _ ~ San Diego of UNB Creek

Site

Fig. 4. Simple linear regression of 24 h data from the Er~ter- omo~gha, b*,testir bioassay experiment showing a) tissue N percent change from initial, b) tissue P percent change fi'om initial, and c) tissue N:P (molar) ratio.

1987; Duar te 1995; Young et al. 1998) ; we observed such mats f loating t h roughou t the estuary while we were sampling. This may also be why E. i,~testinalis tissue N was highest mid-estuary ra ther than at the head of the estuary near the p r imary nu t r i en t source (California Regional Water Quality Cont ro l Board 1997; Boyle and Kamer unpubl i shed data). Cera,tzi'u'm spp. was a t tached to the sed iments and therefore no t able to move t h r o u g h o u t the estuary as E. intestinalis and U. expansa were (Kamer per- sonal observat ion) . T h e highest Cercamiur~z tissue N and P values were measured in samples taken f rom Transect 7 which was the closest t ransect to the m o u t h of San Diego Creek where Ceramiu,m nutrients were measured . Because Cera,~dum is not mo- bile, its tissues ref lected h igher nu t r i en t con ten t closer to the nu t r ien t source as we expected.

Macroalgal tissue N in UNB ( ~ 9 - 4 % dry wt) was comparab le to levels found in o ther field studies. Wheele r and Bj6rnsSter (1999) found tissue N conten t in E. intestinaSs ranging fi-om 2.09-5.11% dry wt, in Ulvafenestrata f rom 2.44-5.48% dry wt, and in Pc~rphyra sp. f rom 2.51-4.55% dr}, wt. In estuaries subject to high nu t r i en t loads, E. intestinaSs had tissue N > 5% dry wt (Soulsby et al. 1985) and Ulva spp. tissue N ranged f rom 1.9-5.0% dry wt (HernSndez et al. 1997). Peckol et al. (1994) repor ted Gracila,ria tihvahiae tissue N v a h e s of 1.7 and 8.5% dr}, wt in algae collected f rom areas with low and high N loading rates, respectively.

Our tissue P v a h e s for E. intestinaSs, U. expansa, mad Cera,mi,u,m spp. were similm- to or lower than others that have been repor ted . In l abora tory exper iments , Bj6rns~ter and Wheele r (1990) repor t - ed tissue P content of 0.S0-0.56% dry wt for E. intestinaSs with and without P en r i chmen t , and 0.21-0.51% dry wt for U. fenestrcatca u n d e r the same conditions. Peckol et al. (1994) r epor ted tissue P of G. tihvahiae at 0.10 and 0.20 % dry wt for algae collected f rom areas with low and high nu t r i en t inpu t s , respect ive ly . W h e e l e r and B i6 rns~ te r (1992) r epor ted tissue P values of field collected E. i~testh~a~is and U. fe~esOata ranging f rom 0.39- 0.78% dry wt and Po~phyra sp. ranged f rom 0.87- 0.86% dry wt. For Erztero~orpha, Ulva, and Cera'm- h,,m, the nmxinaum tissue P values r epo r t ed else- where are well above the lm, els we measured in UNB, suggesting that algae in LrNB may have the ability" to take up m o r e P should it be added to the s3rstem.

Mean N:P ratios of E. intestinalis and U. expa~zsa in UNB were similar to or less than values r epo r t ed in the l i terature for species of g reen macroalgae . Atkinson and Smith (1983) r epor t ed N:P a tomic ratios of 38 for Cladophorca sp., Sg to 67 for species of Hali,meda and 35 to 80 for species of UTva. La- pointe (1997) measured mola r N:P ratios of 57 in


TABLE 5. Ceramic, m, spp. tissue N and P values and tissue N:P (molar) ratios fi'om stumbler and fall 1997. Means are given with standard error and n. Transects where no samples were collected were omitted f rom the table.

Tissue 1N Tissue P Tissue N P

Season Tral:sect Stratul~l Meal: SE i: Meal: S]~ n Meal: SE n

Summer

Fall

2 Upper Lower 2.578 0.290 4 0.233 0.029 4 25.2 3.3 4

6 Upper Lower 3.868 0.365 4 0.213 0.009 4 40.1 2.7 4

8 Upper Lower 2.252 0.287 2 0.230 0.010 2 21.8 3.7 2

1 Upper Lower 3.259 0.443 5 0.200 0 1 30.4 0 1

2 Upper 2.922 0.260 2 0.180 0 1 39.1 0 1 Lower 3.484 0.418 5 0.235 0.010 4 34.6 3.6 4

5 Upper 1.316 0 1 0.110 0 1 26.5 0 1 Lower

6 Upper 3.931 0.283 4 0.248 0.020 4 36.4 5.5 4 Lower 8.988 0.083 5 0.282 0.011 5 81.6 1.9 5

7 Upper 3.168 0.244 2 0.190 0 1 34.1 0 1 Lower 4.492 0.092 5 0.304 0.012 5 32.9 h0 5

Chaeto,mo~pha linu,m and 33 to 44 in species of Cod ium. Larned (1998) found a ratio of 48.5:1 (total N:total P) for Ulvafasdata. The higher N:P ratio found in other green macroalgae suggest that b loom-forming green macroalgae in UNB are not saturated with N, and they may have the ability to take up more N. Fur ther inputs of N to this system may cont inue to fuel algal blooms and cause the current condit ions to worsen unless other factors such as light or temperature become limiting.

The E. intestinatis bioassay may be too sensitive for use in a system as nutrient-rich as UNB appears to be. After 24 h, distance from San Diego Creek only explained 20% of the variability observed, and after 48 h, increases in tissue N were great enough to obscure any potential spatial relationships. Us- ing the bioassay over a shorter time scale, such as a few hours, may provide better insight into nutrient dynamics in highly enriched systems. High variability between replicates at a site may have occurred if nutrients were no t spatially homogenous . Nutrients may travel through the water co lumn in discrete packets and could have affected some, but not all, replicates at one site. T h o u g h only 20% of the variability may be explained by distance, we suggest two mechanisms for the increase in tissue N with proximity to San Diego Creek. Either N was removed from the water co lumn as the flow from San Diego Creek moved through the estuary, or concentra t ions of N entering UNB in the San Di- ego Creek flow were diluted by mixing with tidal waters. If the former occurred, then N was re- tained by the system, either sequestered in the sediments or taken up by the algae and was not simply flushed out to the ocean.

In conclusion, our data indicate that UNB is a highly eutrophic system. UNB experienced high

macroalgal cover during summer and fall seasons with biomass comparable to that of b loom events at other locales. The levels of N in macroalgal tissue collected from UNB indicate that the system is enr iched with N. Yet the algae in UNB may not be saturated with N as indicated by their N:P ratio, which were lower than others that have been reported. Further inputs of N may" be utilized by the algae. Additional nutrients entering the system due to continual growth and development in the wa- tershed have the potential to exacerbate these algal blooms, increasing their magni tude and dura- tion.

ACKNO1,VLED GMENTS

This work was funded by the California Water Resources Cen- ter, the Environmental Protection Agency Water and Watm~ sheds Program #R825381, and a University, of California at Los ,amgeles Academic Senate grant. We thank the Califol"nia De- par tment of Fish and Game for site access and use of their ca- noes. Special thanks are extended to Risa Cohen, Damion Gas- tellum, and Steve Kim for help in the field and to Dm-io Diehl and Jona than Fmgerut for invaluable assistance ruth Fig. 1.

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Received for consideratior~, Fet~'~ary 2, 2000 Accepted fv,r p~blicatior~, March 1 Z 2001

macroalgal bloom dynamics in a highly california estuary

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