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Limnology and terrestrial biology

Temporal variation of specific growthrates for phytoplankton in Lake

Bonney, Antarctica

THOMAS R. SHARP AND JOHN C. PRISCU

Department of BiologyMontana State UniversityBozeman, Montana 59715

Lake Bonney is one of several lakes located in the dry valleysregion adjacent to McMurdo Sound. These lakes experience astrong seasonal variation in irradiance and day length; havepermanent ice caps; strong vertical nutrient, oxygen, and conduc-tivity gradients; and a virtual absence of planktonic grazers(Parker et al. 1982). The permanent 4-meter-thick ice cap of LakeBonney reduces under-ice irradiance to less than 5 percent ofincident and protects the lake from wind-induced turbulence.The ice cover, together with the strong salinity gradient (Spigeletal. 1990) and low advective stream inflow, creates a hydrody-namically stable water column in which the phytoplankton live,an uncommon situation for pelagic ecosystems. The nonturbulentenvironment allows for the phytoplankton genera present tobecome segregated along resource gradients (e.g., light, nutri-ents, and temperature) into distinct strata.

Three distinct phytoplankton assemblages exist within thetrophogenic zone (Priscu et al. 1990; Sharp this issue; Lizotte andPriscu this issue). The assemblages consist of: (i) Cryptophyceaeand Chlorohyceae from just under the ice to 8 meters; (ii) flagel-lated Chrylsophyceae and a small (less than 4 micrometers indiameter) unidentified coccoid from 8 to 16 meters; and (iii)flagellated Chlorophyceae and Chrysophyceae from 16 to 20meters. These phytoplankton assemblages have been shown tohave different photosynthesis-irradiance parameters (Lizotte andPriscu this issue). While earlier studies examined environmentalfactors that influence photosynthesis in the dry valley lakes(Vincent 1981; Parker et al. 1982; Vincent and Vincent 1982; Priscuet al. 1987), none of them focussed on temporal changes inphotosynthetic or growth rates. Vincent (1981) suggested thatphytoplankton growth rates are greatest during the austral win-ter-spring transition when nutrient concentrations should bemaximal and irradience rises above the compensation point forphotosynthesis. To test this hypothesis, we measured phytoplank-ton biomass (chlorophyll a) nominally every 5 to 7 days duringour 1990-1991 (October 1990 to January 1991) and 1991-1992(September to December 1991) sampling seasons to estimatespecific growth rates of phytoplankton biomass from each of thediscrete phytoplankton assemblages in Lake Bonney. Net specificgrowth rates for each phytoplankton assemblage were deter-mined as the change in integrated chlorophyll over the period.Depth integration limits were 4 to 8 meters, 8 to 16 meters, and 16to 20 meters for each of the respective assemblages.

The figure shows temporal distribution of net specific growthrates for each of the phytoplankton assemblages. Net specificgrowth rates in the surface phytoplankton layer showed no clearseasonal trend and tended to fluctuate around zero. Net growthrates in the 8- to 16-meter assemblage were positive during thewinter-spring transition (September to mid-October) and de-creased again in early November (day 300). The rate increasedagain in late November (day 320), remained near zero duringDecember, and increased in January (day 1). The net growth ratesof the 16- to 20-meter phytoplankton layer showed the mostdynamic seasonal variation with negative values until mid-Octo-ber (day 290), followed by a rapid increase and decrease by earlyDecember (day 330); the rates fluctuated around zero after day

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c 1990-91 '1991-92Temporal variation in net specific growth rates (per day) for thesurface (4- to 8-meter), middle (8- to 16-meter), and deep (16- to 20-meter) phytoplankton assemblages during the 1990-1991 and 1991-1992 sampling seasons in Lake Bonney.

1992 REVIEW 257

Average net specific growth rates (per day) for eachphytoplankton assemblage.

Phytoptankton SpecificAssemblage growth rateSurface1990-1991Day 303-11 0.01261991 -1 992Day 252-290 -0.0042Day 290-334 0.0024

Middle1990-1991Day 303-11 0.01621991-1992Day 252-290 0.0142Day 290-334 0.0095

Deep1990-1991Day 303-11 0.01571991-1992Day 252-290 -0.0241Day 290-334 0.0623

330. Net growth rates were averaged (time weighted) over 1990-1991 (day 303-day 11). The rates from 1991-1992 were averagedover two periods; from day 252 to day 290 when there was alight:dark period and from day 290 to day 334 when the sun wascontinually above the horizon. The surface (4- to 8-meters) layerhad the lowest rates averaged over 1990-1991 and the latter partof 1991-1992. Rates were similar in all three assemblages during1990-1991. The surface layer had the highest average rate in 1990-1991. The average rate in the surface (4- to 8-meter) layer wasnegative from day 252 to day 290 in 1991-1992 (table), indicatingthat biomass declined. The middle (8- to 16-meter) layer had thehighest average specific growth rate from day 252 to day 290 in1991-1992. This high rate may have been influenced by settling ofbiomass from the surface layer rather than from productionwithin the middle layer. The lowest and highest average rateswere in deep (16- to 20-meter) layer from day 252 to day 290 andfrom day 290 to day 334, respectively, in 1991-1992.

The increasing growth rate with depth could have resultedfrom increased growth efficiency despite lower light because ofthe compensating effect of higher temperature and nutrientconcentrations with depth. Also, because the highest rates oc-curred within the two deeper layers, they may be illusory, result-ing not from primary production but from the settling of biomassfrom above. The average net specific growth rates for LakeBonney phytoplankton are about four-fold lower than those

previously reported for lake phytoplankton (Forsberg 1985). Therates are also lower than those for phytoplankton in antarcticlakes Fryxell and Vanda (mean growth rates of 0.072 and 2.16 perday were estimated from the data of Priscu et al. 1987. However,the rates reported by Priscu et al. (1987) are based on carbonincorporation into protein, and hence, do not include losses.

In summary, the period of maximal growth in the phytoplank-ton assemblages in Lake Bonney occurs after the onset of continu-ous day light (around 17 October), similar to what Tilzer andDubinsky (1987) hypothesized for phytoplankton in the southernoceans. Our data imply that the hypothesis of Vincent (1981) mustbe revised if it is to apply to all antarctic lakes.

We thank Mike Lizotte, Ben Hatcher, Ron Nuggent, Joe Rudek,and Barb Kelley. This work was supported in part by NationalScience Foundation grant DPP 88-20591.

References

Forsberg, B. R. 1985. The fate of planktonic primary production. Limnologyand Oceanography, 30:807-819.

Lizotte, M. P. and J. C. Priscu. 1992. Photosynthesis-irradiance relation-ships in phytoplankton from the physically stable water column of aperennially ice-covered lake (Lake Bonney, Antarctica). Journal ofPhycology, 28:179-185.

Lizotte,M. P. and J. C. Priscu. 1992. Algal pigments as markers for stratifiedphytoplankton populations in Lake Bonney (dry valleys). AntarcticJournal of the U.S., this issue.

Parker, B. C., G. M. Simmons Jr., K. C. Seaburg, D. D. Cathey, and F. C. T.Allnutt. 1982. Comparative ecology of plankton communities in sevenantarctic oasis lakes. Journal of Plankton Research, 4:271-285.

Priscu, J . C., L. R. Priscu, W. F. Vincent, and C. Howard-Williams. 1987.Photosynthate distribution by microplankton in permanently ice-cov-ered antarctic desert lakes. Limnology and Oceanography, 32:260-270.

Priscu,J.C.,T.R. Sharp, M. P. Lizotte, and P. J. Neale. 1990. Photoadaptationby phytoplankton in permanently ice-covered antarctic lakes: Re-sponse to a non-turbulent environment. Antarctic Journal of the U.S.,25:221-222.

Sharp, T. R. 1992. Phytoplankton ecology in Lake Bonney, Antarctica:Emphasizing temporal variation of growth and loss rates. Master ofScience Thesis, Montana State University.

Spigel, R. H., I. V. Shepard, and J . C. Priscu. 1990. Temperature andconductivity finestructure from Lake Bonney. Antarctic Journal of theU.S., 25:228.

Tilzer, M. M. and Z. Dubinsky. 1987. Effects of temperature and day lengthon the mass balance of antarctic phytoplankton. Polar Biology, 7:3547.

Vincent, W. F. 1981. Production strategies in antarctic inland waters:Phytoplankton eco-physiology in a permanently ice-covered lake.Ecology, 62:1,215-1,224.

Vincent, W. F. and C. L. Vincent. 1982. Factors controlling phytoplanktonproduction in Lake Vanda (77' 5). Canadian Journal of Fisheries andAquatic Science, 39:1,602-1,609.

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