Nutrient excretion by crustacean zooplankton in the deep chlorophyll layer of Lake Superior

Abstract

We hypothesized that the resident and migrating cold-water zooplankton in Lake Superior concentrate beneath the thermocline and elevate nutrient availability via mineralization in the deep chlorophyll layer (DCL). In August 2010 and 2011, diel vertical net tows and incubation experiments were conducted in the western arm of Lake Superior to measure zooplankton distribution and excretion of dissolved nutrients. Limnocalanus macrurus showed the strongest spatial overlap with the DCL, and the broader crustacean community excreted P at 0.145 μg·mg−1·h−1 and N at 0.617 μg·mg−1·h−1 as soluble reactive phosphorus (SRP) and ammonia, respectively. Mysis diluviana displayed strong diel vertical migration into the DCL and excreted P at 0.013 μg·mg−1·h−1 and N at 0.116 μg·mg−1·h−1 as SRP and ammonia, respectively. Based on primary productivity estimates, the zooplankton community has the potential to meet the daily phosphorus demand by phytoplankton in the DCL during late-season stratification. Our excretion estimates indicate a possible coupling between the cold-water zooplankton community and production in the DCL of Lake Superior.