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

Abstract

The abundance and distribution of phytoplankton and zooplankton were followed from January 1993 to March 1994 in Lake Coleridge, a deep oligotrophic alpine lake in the Canterbury high country of New Zealand. Our data suggest physical processes associated with deep mixing and inputs of suspended sediments combined to limit planktonic biomass, diversity, and determine vertical distribution. The lake was strongly stratified in summer with a surface mixed layer of 20 m gradually extending down to 100 m by May. Over 50 taxa of phytoplankton were recorded. Changes in vertical distribution of most planktonic groups coincided with changes in the depth of vertical mixing. Phytoplankton biomass increased through summer with a diatom‐dominated peak in autumn. Cell numbers however, peaked in spring and were dominated by small chlorophytes. Phytoplankton photosynthetic production was highest in autumn 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 spring phytoplankton peak which occurs in a number of other temperate lakes. Bacteria and pico‐phytoplankton biomass peaked in winter following the phytoplankton peak suggesting they were relying at least in part on scenescing phytoplankton cells for dissolved organic carbon. Diversity of ciliated protozoa was low (11 taxa), Cladocera were absent, and the only crustacean zooplankton found in significant numbers was Boeckella hamata. Ciliated protozoan abundance was low (maximum 4600 litre 1) and peaked in summer (January‐March). Copepod nauplii peaked in August and peaks in abundance of copepodites and adults followed in September and November‐January respectively. Only two generations occurred during the year. The peak in copepod biomass followed phytoplankton peaks but lagged 1–2 months probably as a result of slow growth rates and the time for response to phytoplankton peaks.