Population dynamics and biogeochemical significance of Limacina helicina antarctica in the Scotia Sea (Southern Ocean)
Limacina helicina antarctica is a common part of the Southern Ocean zooplankton community but little is known about its life cycle. Here we determine the population structure and standing stock biomass of this species in the Scotia Sea region and use this information to derive estimates for rates of growth, mortality and secondary productivity. Three non-overlapping cohorts were present in the size–frequency distribution, a G2 generation (0-year) with a modal peak at 0.3 mm shell diameter, a G1 generation (1-year) with a modal peak at 2.7 mm and a G generation (2-year) cohort with sizes between 4 and 10 mm. We surmise that at least some L. helicina ant. are capable of living 3 years or more, growing at an average rate of 0.01 mm d−1. Mortality rates were 0.01 d−1 or 3.83 year−1, with 2% of individuals surviving beyond 1 year of age and 0.05% beyond 2 years of age. Standing stock biomass was 178 mg DW m−2 or 32 mg C m−2, divided between 23 mg Corg m−2 and 9 mg Cinorg m−2. The inorganic fraction had a calcium carbonate composition equivalent to 72 mg CaCO3 m−2. Maximum daily productivity during the summer was 1.8 mg C m−2 d−1, made up of 1.3 Corg mg m−2 d−1 and 0.5 mg Cinorg m−2 d−1, and equivalent to 4.2 mg CaCO3 m−2 d−1. The mass specific growth rate (P:B d−1) was 0.06 within the summer period. Growth and production appeared to continue over the autumn and winter months at rates almost equivalent to those in summer. We propose that autumn peaks in sedimenting L. helicina ant., reported by other studies, do not appear to be the consequence of the termination of the life-cycle but are more likely to result from a combination of environmental and behavioural factors.