Mortality, growth, and grazing impact of a cohort of bivalve larvae, Mytilus edulis L.

Abstract

A cohort of bivalve larvae, mainly Mytilus edulis, was followed during its residence in the plankton of the Isefjord, Denmark, and the mortality and growth of the larvae, as well as their grazing effects on the phytoplankton have been estimated.

The pelagic stage lasted one month. The initial number of larvae was about 3000 · 1⁻¹, and the daily mortality was about 13 %. The mean growth rate amounted to 13–16% of the body mass daily, and the net growth efficiency was 60 %.

The cohort of bivalve larvae cleared 40–50 % of the surrounding water daily of small food particles, probably mostly flagellates. The phytoplankton production at the time the bivalve larvae appeared in the plankton was dominated by diatoms that were not accessible as food to the larvae. It is concluded that dense populations of bivalve larvae may exert heavy grazing pressures on the ultraplankton without directly affecting the populations and primary production of larger phytoplankton organisms.