Abstract
Inputs of nutrients and organic matter to wetlands from catchments influence water quality, which, in turn, determines the potential productivity of a wetland. While there are many studies of the effects of catchment development on water quality and aquatic communities in lakes, few studies include other wetlands. As these other wetlands are likely to be more affected by allochthonous inputs, littoral vegetation and autochthonous generation of organic matter, my aim was to determine the major influences on pelagic communities in a range of wetland systems.
Influences of catchment land use on water quality and the pelagic food web were examined in 45 wetlands representative of a range of wetland environments in Otago, including swamps and ponds, shallow lakes, riverine wetlands, estuaries, reservoirs and deep lakes. The pelagic zones of 40 wetland sites were sampled once in February-March 1999 (autumn), 15 were re-sampled in October 1999 (spring), when five additional sites were also sampled. Catchment variables included size and slope, wetland size and the percentage of land in the catchment in bare ground, indigenous forest, inland water, inland wetlands, planted forest, pasture, scrub, tussock, urban or urban open space. Water quality variables, or physicochemical measurements, included total phosphorus, total nitrogen, total dissolved phosphorus, total inorganic nitrogen, dissolved organic carbon, turbidity, Secchi depth, total suspended solids, water colour, chlorophyll a, pH, temperature and conductivity. The pelagic food web was sampled, including biomass, abundance, and identification to genus of phytoplankton, biomasses of picophytoplankton, heterotrophic bacteria and nanoflagellates, biomass and identification to genus or dominant group of ciliates, and abundance and identification to species of crustacean zooplankton. Relationships among catchment variables, water quality variables and the pelagic food web were determined using multivariate analysis and correlation analysis.
Increased development of pasture, exotic forestry and urbanisation in a catchment had negative effects on water quality, in comparison to unmodified catchments containing native vegetation communities. In turn, the biomass and composition of the pelagic community related closely to catchment modification, via physicochemical attributes of the wetland. Deep lakes were the most oligotrophic wetlands and swamps and ponds were the most eutrophic.
Picophytoplankton and the cladoceran, Bosmina meridionalis were related positively to unmodified catchments, low trophic status of a wetland and deep lakes. Other components of the microbial food web, phytoplankton, copepods and Daphnia carinata were linked hierarchically to more intensive land use in the catchment, higher wetland trophy, swamps and ponds. A ciliate genus, Urocentrum, appeared to be detrimentally affected, and phytoplankton diversity reduced, by wetland catchment development and increases in wetland trophy.
Components of the pelagic food web were tightly correlated across adjacent trophic levels. Heterotrophic bacteria appeared to be a resource for heterotrophic nanoflagellates. Picophytoplankton populations might either be suppressed by ciliate grazing, or detrimentally affected by eutrophication. Small ciliates appear to consume other microbial food web components, while larger ciliates may depend more on phytoplankton. Copepods may be relying on consumption of ciliates. Cladocerans did not appear to depend on this resource to the same extent as copepods. Populations of B. meridionalis and Ceriodaphnia dubia were negatively related to the larger cladoceran, D. carinata.
Seasonal effects were apparent only at the level of zooplankton, the highest trophic level studied. B. meridionalis and C. dubia were more abundant in autumn than spring, while the reverse was true of D. carinata.
This study revealed relationships within pelagic food webs in a range of wetland systems. While resource supply appeared to be the foundation of relationships between aquatic organisms, top-down effects of predation in the food web could not be dismissed. The potential of organisms such as picophytoplankton, ciliates and phytoplankton to be indicators of aquatic ecosystem health has been revealed or strengthened by this study. This research provides evidence of the influence of land use and geographical features on water quality and pelagic communities of wetlands in Otago.