Environmental conditions and overwintering strategies of planktonic metazoans in and below coastal fast ice in the Gulf of Finland (Baltic Sea)

Abstract

A field study was conducted in Santala Bay with weekly samplings during February and March 2000. Ice thickness was 20–28 cm, snow cover 0–1 cm. The under‐ice water column was stratified with a cold (−0.3–0.2°C) and less saline (S = 2.1–4.9) interface layer. Concentrations of particulate organic carbon (0.5–5.8 mg POC l ) and algal pigments (0.3–18.2 μg chlorophyll a l ) were higher in the ice than in the water (0.2–0.5 mg POC l , 1.6–7.1 μg chlorophyll a l ) and peaked mostly in the bottom part of the ice. The thin ice and almost lacking snow cover had favoured an early ice‐algal and phytoplankton bloom. The diversity of metazoans was low, with six species in the ice and eight species in the under‐ice water. The rotifer Synchaeta cf. littoralis dominated both in ice and water, with maximum abundances of 230 individuals l in the bottom part of the ice. Rotifer eggs were also observed in the ice. Baltic sea ice seems to be a suitable habitat for rotifers. Nauplii and copepodids of the calanoid Acartia longi...