International Conference on the Status and Future of the

World‘s Large Rivers

11-14 April, 2011 Vienna

Pingram, M.1, Collier, K.1,2, Hamilton, D.1, Hicks, B.1 and David, B.2 1Centre for Biodiversity and Ecological Research, University of Waikato, Hamilton, New Zealand, e-mail: map27@waikato.ac.nz, 2Environment Waikato, Hamilton, New Zealand

Food webs in New Zealand’s longest river Spatial patterns of 13C and 15N stable isotopes

Karapiro Dam (0 km)

Waipa River (52 km)

Opuatia Stream (WL, 91 km)

Whangape Stream (RL, 88 km)

Waahi Stream (RL, 69 km)

Mangawara Stream (DW, 61 km)

Mangatawhiri River (MW, 107 km)

Whangamarino River (WL & RL, 105 km)

Hoods Landing (140 km)

Methods We used stable isotopes (δ13C and δ15N ) to investigate spatial patterns of basal resource use by consumers in the river and contributions from its major tributaries. We sampled sites above and within major tributaries during Autumn and Spring 2009 (Fig. 2) and collected food web components from different trophic levels and guilds, including: • basal resources such as seston and biofilms • primary consumers such as snails, amphipods and grey mullet (Mugil

cephalus) • secondary consumers such as damselfly larvae, common bully

(Gobiomorphus cotidianus), common smelt (Retropinna retropinna) and gambusia (Gambusia affinis)

• Larger predators such as shortfin eel (Anguilla australis)

Introduction Globally, large rivers present a challenging environment to study spatial patterns of carbon flow and trophic ecology. New Zealand’s longest river is the Waikato (442 km in length), with a mean annual discharge of 450 m3s-1, draining a catchment >14,000 km2 (Fig. 1). The catchment and its floodplain have been significantly altered, mostly for agriculture, forestry, flood protection and urban development. The overall aims of our research are to: • build a detailed understanding of food web structure and energy

flow in the lower Waikato River • inform and guide restoration initiatives • contribute to global research of large river ecosystems

Results δ13C of littoral and benthic consumers such as amphipods,

shrimps, common bully and gambusia becomes increasingly depleted with increasing distance downstream. A similar pattern is evident for basal resources such as fine seston and aquatic macrophytes (Fig. 3a).

In contrast, biofilms below the Waipa are more depleted in δ13C than those above or in the tidal section. This pattern is also seen in biofilm grazers such as snails (Fig. 3a).

Although longitudinal patterns in δ15N are less clear, several food web components become enriched (e.g. amphipods) or depleted (e.g. biofilms) below the Waipa River (Fig. 3b).

Mobile fish species such as grey mullet, shortfin eel and common smelt generally show less site specific stable isotope ratios. Benthic and littoral species appear to represent local signatures, particularly when comparing between tributaries and the river (Fig. 4).

Acknowledgements This study was funded by Environment Waikato and the University of Waikato. We thank Warrick Powrie and Dudley Bell of the Department of Biology for technical assistance with field sampling. Isotope samples were analysed by the University of Waikato Stable Isotope Unit by Anjana Rajendram and Judy Hoult.

Fig. 2: The lower Waikato River (■ above the Waipa River, ■ below the Waipa River and ■ tidally influenced sections) and major tributaries. Parentheses indicate tributary type (WL – Wetland, RL-Riverine lake, and DW – Drained wetland) and distance from Karapiro Dam. Photos courtesy of Warrick Powrie.

Fig. 1: Geographic location of the Waikato River, with the lower river marked in yellow.

0 10 20 30Kilometres

N

δ1

5N

(o/o

o)

(b)

(a)

Fig. 3: (a) δ 13C and (b) δ15N of selected consumers and basal resources sampled from the lower Waikato River during Autumn and Spring 2009. ■ above the Waipa River ■ below the Waipa River ■ tidal influenced Error bars represent ± 1 S.E.

Fig. 4: Mean δ 13C and δ15N ratios of fish collected from major tributary sites during Autumn and Spring 2009. Boxes represent the mean ± 1 S.E. of fish collected from the lower Waikato River.

δ1

3C

(o/o

o)

Species

Tributary

δ1

5N

(o/o

o)

Common bully Gambusia Grey Mullet Shortfin eel Common smelt

Waipa Mangawara Waahi Whangape Opuatia Whangamarino Mangatawhiri

δ13C(o/oo)

Aq

uat

ic

mac

rop

hyt

es

Fi

ne

se

sto

n

B

iofi

lms

Sh

rim

ps

A

mp

hip

od

s

Snai

ls

C

om

mo

n

Bu

lly

D

amse

lfly

La

rvae

Study Site A series of hydroelectric dams has been constructed on the river. The furthest downstream dam, at Karapiro, acts as a barrier to natural upstream movement of aquatic flora and fauna. Our study focuses on a 150 km reach of the lower Waikato River from Karapiro to the sea. The lower river is fed by several important tributaries originating from riverine lakes and wetlands of varying ecological integrity. The largest of these is the Waipa River which contributes large amounts of sediment and nutrients, particularly during flooding.

Discussion Stable isotope ratios of food web components can be related to

potentially unique water chemistry and hydrogeology of tributary systems.

These data will allow us to quantify the energy flow in the lower Waikato river food web. Preliminary results indicate that autochthonous carbon sources such as biofilms and suspended algae are likely to be important basal resources.

Future data from diet analyses and otolith microchemistry will complement existing information and strengthen food web models, particularly for valued taxa such as shortfin eel and grey mullet.

Spatial patterns in the isotopic signatures of potentially important food web components should be accounted for when undertaking food web modelling using stable isotopes.