Robert H. Garritt

Multiple Stable Isotopes Used to Trace the Flow of Organic Matter in Estuarine Food Webs

The use of a combination of the stable isotopes of sulfur, carbon, and nitrogen allows the flow of organic matter and trophic relations in salt marshes and estuaries to be traced while eliminating many ambiguities that accompany the use of a single isotopic tracer. Salt-marsh grasses take up the isotopically light sulfides formed during sulfate reduction, and the transfer of this light sulfur through the marsh food web is illustrated with data on the ribbed mussel (Geukensia demissa) from various locations in a New England marsh. The multiple isotope approach shows that this filter feeder consumes both marsh grass ( Spartina) detritus and plankton, with the relative proportions of each determined by the location of the mussels in the marsh.

Sulfur and Carbon Isotopes as Tracers of Salt-Marsh Organic Matter Flow

Stable isotopes of sulfur and carbon were used to trace the dominant flows of organic matter from producers to macroconsumers in Great Sippewissett Salt Marsh on Cape Cod. Spartina alterniflora and sulfur-oxidizing bacteria were found to assimilate isotopically light sulfides produced via sulfate reduction, and this light sulfur was detected in consumers. In contrast, phytoplankton and upland plants assimilate isotopically heavier SO42- with little or no fractionation. A dual-isotope approach using both 613C and 634S showed that Ilyanassa obsoleta and Fundulus heteroclitus depend very heavily on Spartina detritus, while filter feeders such as Crassostrea virginica and Geukensia demissa depend on a mixture of plankton and Spartina detritus. Spartina detritus and plankton were both shown to be much more important as organic matter sources for marsh macroconsumers than either sulfur-oxidizing bacteria or organic matter derived from terrestrial inputs.

Benthic Metabolism and Nutrient Cycling Along an Estuarine Salinity Gradient

Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m−2) but especially so in the sandy marine sediments (172 mg chlorophyll a m−2). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH4+ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest at the marine site (13 mmol O2 m−2 d−1 and 21 mmol TCO2 m−2 d−1) and highest in the mid estuary (130 mmol O2 m−2 d−1 and 170 mmol TCO2 m−2 d−1) where clam densities were also high. N2O and CH4 fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO43− flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO43− flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH4+ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH4+ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.

Ecosystem respiration and organic carbon processing in a large, tidally influenced river: the Hudson River

We estimated whole-ecosystem rates of respiration over a 40-km stretch of the tidally influenced freshwater Hudson River every 2 to 3 weeks from May through November. We measured in situ concentrations of oxygen over depth at dusk and dawn at 10 stations spaced over this interval. The use of multiple stations allowed for the consideration of the influence of tidal advection of water masses. Respiration was estimated from the decrease in oxygen overnight with a correction for diffusive exchange of oxygen with the atmosphere. We estimated this flux of oxygen to or from the atmosphere using the measured oxygen gradient and a transfer velocity model which is a function of wind velocity.Integration of the data for the period of May through November yields an estimate of whole-ecosystem respiration of 591 g C m−2 (S.E. = 66). That the standard error of this estimate is relatively low (11% of the estimate) indicates that the use of multiple stations adequately deals with error introduced through the advection of water between stations. The logarithm of average daily respiration rate was correlated with average daily temperature (p = 0.007;r2 = 0.62). We used this temperature-respiration relationship to derive an estimate of the annual respiration rate of 755 g C m−2 yr−1 (S.E. = 72). This estimate is moderately sensitive to the estimated flux of oxygen between the atmosphere and water; using the lower and upper 95% confidence limits of our model relating the transfer velocity of oxygen to wind speed gives a range of annual respiration estimates from 665 g C m−2 yr−1 to 984 g C m−2 yr−1.The river is strongly heterotrophic, with most respiration driven by allochthonous inputs of organic matter from terrestrial ecosystems. The majority of the allochthonous inputs to the river (over 60%) are apparently metabolized within the river. Any change in allochthonous inputs due to changes in land use or climate patterns would be expected to alter the oxygen dynamics and energy flow within this tidally influenced river.

Population Size and Site Fidelity of Fundulus heteroclitus in a Macrotidal Saltmarsh Creek

Fundulus heteroclitus (mummichog) is the numerically dominant fish species found in salt marshes from Florida to Nova Scotia. These areas exhibit a wide range of fluctuation in environmental conditions such as temperature, salinity, and tidal range. Mummichogs are known to be eurythermal and euryhaline, but there is little information about the influence of tidal range on population dynamics. Our purpose was to determine whether extreme tides (>3 m) in a tidal creek system would affect the population density and site fidelity of F. heteroclitus. Both population density and site fidelity were measured by mark-recapture experiments in a primary saltmarsh creek (named here Sweeney Creek) off the Rowley River in Plum Island Estuary, a macrotidal (>3 m) estuary (Fig. 1). Both branches were approximately 300 m long from the confluence to the upland end. From 18 June to 3 I July 1998, temperature (data logger), salinity (refractometer), and fish abundance were monitored. Six, 6.35mm-mesh minnow traps were set in the center and randomly apart on both branches at around 180 m (East) and 120 m (West) from the confluence (Fig. 1). Specimens of F. heteroclitus 2 40 mm caught in the traps were counted and marked yellow (West) and blue (East) by injection of acrylic dye under the skin on the left dorsal side, then released. Population size was estimated using the Bailey model (1). Site fidelity was measured as the number of marked fish recaptured in different locations. Opposite-colored fish found in a marking zone were recorded and remarked on the opposite dorsal side with the color of the branch in which captured. To determine how far and by what path fish moved, we set traps in three locations. In the secondary creek, two traps were set at five stations 22 to 106 m from the confluence. Nine traps were set in the mosquito ditches, and traps were set upstream of the marking locations. The pannes and secondary creek were also seined. Time intervals between mark and recapture were variable, averaging a 14.75 hour catch per unit effort. Temperature and salinity were within the normal range for F. heteroclitus. Water temperature averaged 20°C at night and 26°C during the day for both branches. Salinity ranged from 8 to 25 ppt for the East branch, and 10 to 26 ppt for the West.

From Watershed to Estuary: Assessment of Nutrient Loading, Retention, and Export from the Ipswich River Basin

Childs River and Quashnet River, combined with the absence of such spikes at Sage Lot and Flat Ponds, suggests input by plumes from septic tanks. Mean NI&+ concentration in groundwater varied over a relatively small range across the three sites (Fig. 1, middle panels). NH4+ contributed less than 5% of the mean dissolved inorganic nitrogen (DIN) at Childs and Quashnet Rivers. Most of the inorganic nitrogen, therefore, enters these two estuaries in the form of N03-. Nitrogen from septic tanks and soils enters the aquifer in reduced forms, such as NH.,+ [untreated septic water contains up to 8000 PM NH4+ (J. McClelland, unpub. data)], which are oxidized to N09in the aquifer before reaching the estuary. Mean POd3+ concentrations were relatively low and similar in the three estuaries (Fig. 1, right panels). One would not expect POd3+ to vary across the sites, despite a gradient in residential area, because POd3+ is readily adsorbed by soils. Groundwater data from Waquoit Bay suggest that differences in land use can have a major effect on the concentrations of nitrogen entering a shallow estuary. To estimate the loading from groundwater to receiving estuaries and to estuarine systems, such as Waquoit Bay, concentration data such as those reported in this paper must be converted to flux by estimating actual water and nutrient transport. Calculated values of total system loading will be useful in deciding how to manage coastal estuaries threatened by eutrophication. This work was supported by Waquoit Bay Land Margin Ecosystems Research and a Research Experience for Undergraduates grant. Special thanks to Lori Saucy for operating the autoanalyzer.