Relationships between reproduction in suspension-feeding hard clams mercearia mercenaria and phytoplankton community structure

We hypothesized that the decline in hard clam Mercenaria mercenaria populations since the 1970s in many of the south shore bays of Long Island, New York, USA, was due to changes in reproduction caused by variations in phytoplankton available as a food resource. We measured in 2000 and 2001 the reproductive condition of female clams from 5 sites in south shore bays of Long Island and from 2 sites in New Jersey. Phytoplankton composition was measured as the concentration of total and size-fractionated chlorophyll a (chl a), particulate organic carbon (POC), and nitrogen (PON), and the abundance of the harmful picoplankter Aureococcus anophagefferens. Timing of peak hard clam reproduction was nearly identical across sites, and was the same as in archived samples of clams collected in 1978 and 1979. Hard clam condition index differed between sites, with poorest condition being exhibited at the 3 Great South Bay (GSB) sites. There were also appreciable differences in relative reproductive output between locations, with clams again from the 3 GSB sites having the lowest estimated egg production. The central and eastern GSB had the highest levels of total chl a, POC, and PON, however, of any south shore bay, although this was mainly contributed by cells <2 pm, which have low nutritional value for adult hard clams. In addition, the 3 sites in GSB suffered brown tide blooms during 2000 and 2001, which are known to inhibit bivalve feeding. We conclude that changes in the composition of the phytoplankton community at some locations may be reducing hard clam reproductive output.