Ecosystem considerations about Mya arenaria in urbanizing estuaries

Background/Question/Methods:

The Plum Island estuary (PIE) in Massachusetts is a productive ecosystem beginning to show signs of eutrophication in its upper, oligotrophic regions. Filter-feeding bivalves, such as clams, may help limit the effects of eutrophication by filtering phytoplankton from the water column. This research aims to identify the effectiveness of the softshell clam, Mya arenaria, in controlling the phytoplankton abundance in PIE. We examine the relationship among clam distribution, biomass, food resources and growth rates, phytoplankton biomass and distribution, and physico-chemical conditions in the water column. Water samples for particular organic matter (POM) and Chlorophyll-a concentration, potential food/detritus resources (Spartina patens, Spartina alterniflora, and Typha arigustifolia ) and approximately 40 clams (2" and 4") were collected monthly from May to November in 2010 and April 2011 at eight sites along the length of the estuary. The eight sites represented a gradient across food resource base, salinity, and terrestrial/oceanic influence. Chlorophyll-a concentration was estimated through fluorometry. Stable isotope analyses of δ¹³C, δ¹⁵N, δ³⁴S and δD were performed on adductor muscles of clams, plants and POMs. Non-metric multidimensional scaling (NMS) analyses were carried out on the isotopic composition of clams to investigate dissimilarity in food resources among sites. Internal annual growth ring analyses on approximately 20 shells from each site were used to estimate clam growth.

Results/Conclusions:

Results show for the middle and lower estuary similarity in phytoplankton density, food resources and clam growth. The isotopic values of clams, plants and POM reflect that clams in the middle and lower regions primarily depend on marine food resources. When compared to the middle and lower estuaries, the upper region shows: 1) much higher phytoplankton density during the period of most rapid clam growth (May to October); 2) higher δ¹⁵N and δD and lower δ¹³C and δ³⁴S concentrations, which indicates clams rely on diverse terrestrial and/or marsh resources as well as marine resources; 3) dissimilarity in food resources based on the NMS analyses; 4) significant decrease in size and biomass of clams (= -0.17*S (site) + 15.42,  p<0.05).  In summary, while patterns of isotopic composition and phytoplankton density suggest that the upper region has better conditions in food supply, the ring analyses show a significant decrease in clam biomass, size and growth in the upper estuary. These results indicate that salinity is a dominant factor controlling clams growth and determining the effectiveness of softshell clams, Mya arenaria, in controlling phytoplankton population in PIE.