Coastal ecosystems are inherently complex and potentially adaptive as they respond to changes in nutrient loads and climate. We documented the role that carbon stable isotope (δ 13 C) measurements could play in understanding that adaptation with a series of three Ecostat (i.e., continuous culture) experiments. We quantified linkages among δ 13 C, nutrients, carbonate chemistry, primary, and secondary production in temperate estuarine waters. Experimental culture vessels (9.1 L) containing 33% whole and 67% filtered (0.2 µm) seawater were amended with dissolved inorganic nitrogen (N) and phosphorous (P) in low (3 vessels; 5 µM N, 0.3 µM P), moderate (3 vessels; 25 µM N, 1.6 µM P), and high amounts (3 vessels; 50 µM N, 3.1 µM P). The parameters necessary to calculate carbonate chemistry, chlorophyll-a concentrations, and particulate δ 13 C values were measured throughout the 14 day experiments. Outflow lines from the experimental vessels fed 250 ml containers seeded with juvenile blue mussels (Mytilus edulis). Mussel subsamples were harvested on days 0, 7, and 14 and their tissues were analyzed for δ 13 C values. We consistently observed that particulate δ 13 C values were positively correlated with chlorophyll-a, carbonate chemistry, and to changes in the ratio of bicarbonate to dissolved carbon dioxide (HCO − 3 :CO 2 ). While the relative proportion of HCO − 3 to CO 2 increased over the 14 days, concentrations of each declined, reflecting the drawdown of carbon associated with enhanced production. Plankton δ 13 C values, like chlorophyll-a concentrations, increased over the course of each experiment, with the greatest increases in the moderate and high treatments. Trends in δ 13 C over time were also observed in the mussel tissues. Despite ecological variability and different plankton abundances the experiments consistently demonstrated how δ 13 C values in primary producers and consumers reflected nutrient availability, via its impact on carbonate chemistry. We applied a series of mixed-effects models to observational data from Narragansett Bay and the model that included in situ δ 13 C and percent organic matter was the best predictor of . In temperate, plankton-dominated estuaries, δ 13 C values in plankton and filter feeders reflect net productivity and are a valuable tool to understand the production conditions under which the base of the food chain was formed.

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We used growth rates of juvenile winter flounder Pseudopleuronectes americanus, to assess habitat quality in 3 of Rhode Island's coastal salt ponds that had differing levels of nutrients and human development. In each pond, 1 m 2 cages were placed in vegetated and unvegetated habitats and growth rates of individually marked fish were measured in three 10 to 15 d experiments from 4 June to 7 July 1997. Water temperature, salinity, dissolved oxygen, and benthic food were also measured. Stable isotopes of C and N were measured in experimental and wild fish. Growth rates were 0.06 to 0.76 mm d in Point Judith Pond and 0.21 in Ninigret Pond) were similar to the average of the 2nd and 3rd experiments (0.24 mm d -1 ). Growth rates were the same in vegetated and unvegetated sites. They were also the same in Point Judith and Ninigret ponds but lower in Green Hill Pond. An ANCOVA suggested that Green Hill's lower rates were caused by its higher temperatures, particularly during the 3rd experiment. Benthic food was similar in the different ponds, different habitat types, and in cores taken inside and outside cages. Categories of food consumed by fish were not affected by the presence of vegetation in a cage, although food consumed did differ from pond to pond. Amphipods were the preferred food in all ponds; fish consumed proportionately more amphipods and fewer polychaetes in Ninigret Pond than in the other ponds. Values of δ 15 N in the fish varied with the degree of development in the watershed but not with total nitrogen in the water column. The results of this study indicate that growth rates of fish can be used as indicators of habitat quality. KEY WORDS: Fish habitat · Habitat quality · Estuaries · Fish growth · Stable isotopesResale or republication not permitted without written consent of the publisher

Stable carbon isotope ratios were measured in archived striped bass, Morone saxatilis (Walbaum), scales to identify changes in the feeding behaviour of this species over time. Striped bass tissue and scale samples were collected from Rhode Island coastal waters during 1996 and archived scale samples (1982–1997) were obtained from Chesapeake Bay. Known striped bass prey items were also collected from Chesapeake Bay and analysed for δ13C. A significant correlation was observed between carbon isotope ratios in striped bass scales and muscle tissue (r2 = 0.52; P < 0.05). Carbon isotope ratios were enriched (less negative) in scales relative to muscle tissue by about 3‰. Carbon isotope ratios in archived striped bass scales from Chesapeake Bay increased significantly from −16.7 ± 0.2‰ in 1982 to −15.1 ± 0.3‰ in 1997. Benthic species, especially invertebrates, were isotopically enriched relative to pelagic fish species collected from the main‐stem of Chesapeake Bay. Prey samples collected from riverine locations within Chesapeake Bay were isotopically depleted relative to those collected in the open portion of the Bay. The changes in the carbon isotope ratios of the striped bass scales could be related to changes in the relative proportions of pelagic and benthic food items in the diet of striped bass or to changes in the feeding locations of this species. In either case, there have been changes in the feeding behaviour and/or relationships of the striped bass between 1982 and 1997. Such changes may be related to changing ecological conditions within the estuary, which could influence the health of Chesapeake Bay striped bass.