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Boston University Marine Program, Marine Biological Laboratory, Woods Hole, Massachusetts 02543
Scallops (Argopecten irradians) feed on particulates in estuaries, and their growth and survival may depend on the quality and quantity of food particles available (1,2). To a significant degree, particle supply in shallow estuaries such as those on Cape Cod depend on rates of land-derived N load (3). Linkages between estuarine organisms and terrestrial loadings have been studied in various ways, including stable isotopic techniques. Isotopic fractionation leads to detectable shifts created by microbial transformations, trophic steps, as well as to differences due to source of the N (4,5).
In this paper we apply isotopic analyses and experiments with introduced scallops to define the rate at which scallops acquire the signature of the estuary in which they are located; we examine whether scallop tissues differ from the signatures of pseudofeces and feces ejected by scallops, and whether differences in N-loading rates and sources to different estuaries result in corresponding differences in the signature of scallops within the estuaries. Finally, we use results of the introduced scallop experiments to see if differences in 
15N signature acquisition are related to differences in growth or survival of the scallops.
We compared the acquisition of 15N signatures by scallops incubated in two estuaries of Waquoit Bay, Cape Cod, receiving different N inputs. Childs River (CR) has a loading rate of 601 kg N ha-1 y-1. Sage Lot Pond (SLP) has a loading rate of 14 kg N ha-1 y-1. The difference in N load between these estuaries is due to different levels of urbanization in their watersheds, and the differences in wastewater contributions to these two estuaries result in different isotopic signatures in the N entering the estuaries from land (5,6). Juvenile scallops (40–50 mm) were obtained from Taylor Seafood, Fairhaven, Connecticut. In each estuary we placed four plastic-coated wire cages, each containing 20 scallops. Cages were secured 10 cm above the sediment surface in 1 m of water at mean low tide.
To monitor the acquisition of the 15N signature in tissue and ejecta over time, we removed one cage of scallops from each estuary on days 3, 6, 12, and 24. Animals were immediately placed in filtered seawater for 24 hours to clear their guts. Feces and pseudofeces were filtered through pre-ashed, 7-µm Whatman GF/F filters. Scallop tissue was dissected from the shell and dried at 60 °C overnight. Ejecta were acidified to remove carbonates, and samples not collected on filters were homogenized.
We determined the 15N signatures of potential food sources, particulate organic matter in water (POM, or seston) and sediments. In each estuary, we sampled the water column and sediments near the cages on days 0, 3, 6, 12, and 24. Water column samples were processed in the same manner as ejecta. The top 1 cm of sediment was sampled using a 5-cc syringe as a corer. We combined four sediment cores for each sample. Sediment samples were acidified and homogenized. All samples were analyzed using a Europa Scientific Integra mass spectrometer at the University of California-Davis.
To determine scallop growth over time, length of shells of animals from each cage were measured with vernier calipers accurate to 0.1 mm. The number of dead scallops per cage were counted on each collection day.
The 15N values of tissue from scallops grown in each estuary were initially 9.23% and during the course of the field incubation approached 15N values of POM in water and sediments, corrected by an expected trophic fractionation of 3% (4) (Fig. 1A, B). For example, if scallops in CR were feeding only on sediments, we extrapolate that the scallops, at the measured rate of change in tissue signature, would converge on the mean sediment signature (corrected by a 3% trophic fractionation) in 93 days. Similarly, if the scallops were feeding on only seston, the convergence would take place in 60 days. For the scallops in SLP, the convergence time would be shorter: 47 days and 36 days, respectively.
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15N signature of ejecta (feces + pseudofeces) to the 15N signature of food supply from each estuary. Lighter 15N signatures for food in SLP corresponded to lighter 15N signatures in ejecta from the scallops grown in SLP, while heavier 15N food signatures in CR corresponded to heavier ejecta signatures from the CR scallops (Fig. 1C). In both estuaries, the 15N signature of ejecta was equal to or heavier than that of potential food sources (Fig. 1C). In addition, 15N signatures of ejecta were lighter than 15N signatures of tissue in CR [8.75%–9.85% (Fig. 1A)] and SLP [0.07%–9.23% (Fig. 1B)]. The 2%–3% enrichment from food to ejecta agrees with trophic level fractionation reported in the literature. The relative similarity between the 15N signatures of seston and sediments makes it difficult, however, to determine which food source contributed most to the diet of scallops during this study.
The faster rate at which SLP scallops approached predicted 15N signatures of their food sources (Fig. 1A, B) may be related to the faster growth of scallops in SLP (Fig. 1D). SLP scallops grew more quickly and achieved greater length than CR scallops (Fig. 1D). Mean growth rates (from incremental growth data) are 0.24 ± 0.03 mm/day for SLP, and 0.14 ± 0.01 mm/day for CR. In addition, no scallops in SLP died during the study, whereas those in CR reached 20% mortality by day 24 (Fig. 1D). The data suggest that conditions in CR were less favorable for scallops than conditions in SLP. This could be related to lower water quality in CR (7), which could have lowered feeding rate and possibly altered the rate of internal turnover of nitrogen within the scallop tissue.
Scallop 15N signatures moved toward the signatures of their presumed food sources at a rate suggesting they would converge with trophic-shift-corrected 15N food signatures in 1–3 months of feeding. Material ejected by scallops had heavier 15N signatures than potential food signatures but lighter than tissue signatures. The increased wastewater N load in CR coincided with a slower convergence of tissue signatures to trophic-shift-corrected food signatures, lowered growth, and increased mortality.
Thanks to Marci Cole, Gabby Tomasky, Joanna York, and Marshall Otter for technical assistance, and the residents of 71 Childs River Road for providing site access. This work was supported by NSF-Research Experience for Undergraduates Grant OCE-0097498 and the Five College Coastal and Marine Sciences Program’s participation in the Woods Hole Marine Sciences Consortium.
Footnotes
Literature Cited
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