Seasonal variation and distribution of zooplankton in the deglaciating fjord of Glacier Bay National Park, Alaska

Abstract

Seasonal composition and abundance of zooplankton along with their physical and chemical environment were investigated in Glacier Bay, AK. Collections were taken six times annually during 2016 and 2017 and separated into upper (0-50 m) and lower water column (50180 m) strata. We found ~70 zooplankton taxa, with copepods dominating both abundance and biomass. We find that zooplankton concentrations within the bay are often an order of magnitude higher than outside. Zooplankton had species-specific preference for particular habitats, with significant populations of Acartia and Oithona in the upper column, Metridia in the lower column, and Pseudocalanus throughout the water column. Seasonality was clearly evident in the upper column, but less so at depth. Copepod nauplii had highest abundances during the spring, concurrent with the spring phytoplankton bloom. Total zooplankton were highest in the summer but with somewhat different timing between the two years. Highest annual zooplankton abundances were observed during May and July of 2016, while during 2017 the highest observed abundances shifted to July and September. These temporal shifts may be temperature-related as 2016 was generally warmer than 2017 by 1-2 °C. Community composition in Glacier Bay differs slightly in leading species from that of the Gulf of Alaska and adjoining Icy Strait. We suggest bathymetry in the bay is too shallow for Neocalanus and Calanus species to form overwintering populations.