Seasonal changes in plankton and nutrient dynamics and carbon flow in the pelagic zone of a large, glacial lake: effects of suspended solids and physical mixing

The abundance and distribution of phytoplankton and zooplankton were follow ed from January 1993 to March 1994 in Lake Coleridge, a deep oligotrophic a lpine lake in the Canterbury high country of New Zealand. Our data suggest physical processes associated with deep mixing and inputs of suspended sedi ments combined to limit planktonic biomass, diversity, and determine vertic al distribution. The lake was strongly stratified in summer with a surface mixed layer of 20 m gradually extending down to 100 in by May. Over 50 taxa of phytoplankton were recorded. Changes in vertical distribution of most p lanktonic groups coincided with changes in the depth of vertical mixing. Ph ytoplankton biomass increased through summer with a diatom-dominated peak i n autumn. Cell numbers however, peaked in spring and were dominated by smal l chlorophytes. Phytoplankton photosynthetic production was highest in autu mn but specific growth rates were highest in summer as a result of greater light availability combined with warmer temperatures. We suggest that large inputs of suspended sediments and glacial silt., may have limited the spri ng phytoplankton peak which occurs in a number of other temperate lakes. Ba cteria and pico-phytoplankton biomass peaked in winter following the phytop lankton peak suggesting they were relying at least in part on scenescing ph ytoplankton cells For dissolved organic carbon. Diversity of ciliated proto zoa was low (I I taxa), Cladocera were absent, and the only crustacean zoop lankton found in significant numbers was Boeckella hamata. Ciliated protozo an abundance was low (maximum 4600 litre (1)) and peaked in summer (January -March). Copepod nauplii peaked in August and peaks in abundance of copepod ites and adults followed in September and November-January respectively. On ly two generations occurred during the year. The peak in copepod biomass fo llowed phytoplankton peaks but lagged 1-2 months probably as a result of sl ow growth rates and the time for response to phytoplankton peaks.