Increased land-derived nitrogen loads have
eutrophied many estuaries, potentially
changing estuarine features important to
commercially harvested bivalves. To define
how land-derived nitrogen loads affect
bivalves, I compared growth, survival, and
physiological responses of quahogs,
Mercenaria mercenaria, and softshell clams,
Mya arenaria, across estuaries receiving
different N loads.

First, to determine how nitrogen enrichment
affected estuarine features and relate those
changes to food supply and habitat for clams,
I measured isotopic composition, chlorophyll
a, carbon, and nitrogen in seston and
sediment, and other environmental variables
at the sediment-water interface where clams
live and feed. Major effects of nitrogen
enrichment on near-bottom seston and
surface sediment were to 1) increase
microalgal concentrations and reduce carbon
to nitrogen ratios, increasing quantity and
quality of available foods, and 2) reduce
oxygen content in sediments, potentially
reducing habitat quality.

Second, to determine how N loads affected
bivalve growth and survival, I measured shell
growth and survival directly in transplanted
clams and estimated growth throughout life
indirectly in native clams by the von Bertalanffy
growth model. Although growth was limited by
salinity in some estuaries, and low oxygen
concentrations may have reduced survival, the
major effect of N enrichment on clams was
increased growth through increased food
supply.

Third, to link bivalve responses to land-derived
N loads and determine which food sources
were most important to clams, I compared
nitrogen and carbon isotopic signatures in
foods to those in clam tissues. Clams
selected particles primarily from seston, and
nitrogen loading did not change the type of
particles assimilated.

Fourth, to assess nitrogen and carbon
assimilation and turnover in clam tissues in
response to changes in food supply, I
measured changes in nitrogen and carbon
isotopic composition of seston and sediment
and in tissues and egesta of transplanted
clams. Isotopic signatures in clam tissue and
egesta responded rapidly to changes in food
supply, with most change due to growth.
Nitrogen turnover rates were 0.6% per day in
both species.

These results are biologically relevant to
ecology and stock management of clams, and
suggest that the major effect of N enrichment
on clams was increased secondary
production.