The influence of plankton composition and water quality on hard clam (Mercenaria mercenaria L.) populations across Long Island's south shore lagoon estuaries (New York, USA)

We conducted a two-year study to assess how plankton composition and water quality impacts the distribution, densities, condition, growth, biochemical composition and reproductive success of juvenile and adult Mercenaria mercenaria (L.) in Long Island's south shore estuaries (LISSE). Juvenile and adult hard clams were placed in suspended cages at 10 locations ranging from the ocean inlets to locations furthest from inlets in Shinnecock Bay (SB), the eastern-most barrier island estuary of LISSE, and Great South Bay (GSB), the western-most barrier island estuary of LISSE. Phytoplankton community composition, temperature, salinity, dissolved oxygen, and clam growth and condition were monitored bi-weekly. A benthic survey of M. mercenaria densities in both estuaries was also conducted. In both 2004 and 2005, juveniles in central bay locations had significantly faster growth rates, lower mortality rates, and higher lipid content relative to sites closest to the inlets. Adult hard clams closest to the Fire Island inlet also had significantly lower condition indexes compared to mid-bay stations and densities of wild M. mercenaria populations in both estuaries were lower near inlets compared to locations further from inlets. In addition to substantial spatial differences within each estuary, differences were also observed between the embayments as juvenile clams in SB grew approximately twice as fast as those in GSB and adults in SB had significantly greater condition indexes than clams in GSB. Instantaneous juvenile growth rates were highly correlated to temperatures below 24 °C (p < 0.0001) and were also significantly correlated with several indicators of suspended food quantity and food quality (centric diatoms, phytoplankton cells > 5 μm, and dinoflagellates (inverse correlation)) which co-varied independently of temperature. In sum, these results suggest tidal exchange in LISSE promotes a water quality regime (cold water, with low food concentration) which would reduce the growth of juvenile clams and the overall reproductive success of adult hard clams located near newly-formed ocean inlets. However, increased exchange for regions furthest from inlets could enhance juvenile clam growth rates by reducing summer peak temperatures (> 24 °C) and densities of poor food sources (dinoflagellates).