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Virginia Sea Grant funded research

Evaluation of nursery habitats: Effects of carbon and nitrogen sources on juvenile fish growth in Chesapeake Bay

Cynthia Jones and Robert Dias (Old Dominion University)

Our objective is to link the influence of C and N source to fish growth in our model system of spotted seatrout and seagrass nurseries, thus accomplishing one of our long-term goals of quantifying the value of seagrasses as nurseries in Chesapeake Bay. In 2007, using stable isotopes, we seek to evaluate differences in sources of C and N in the major seagrass habitats in the lower Bay. We will simultaneously evaluate C and N from muscle of resident-juvenile spotted seatrout from these habitats. We will obtain otolith-increment measurements from our NSF research to evaluate individual-fish growth correlates. In 2008, we will evaluate sources of C and N  and fish muscle as in 2007 and determine whether these can be correlated to environmental factors for both years. We will obtain otolith-increment measurements from our NSF research to evaluate individual-fish growth correlates.

Methods that we will employ are already well developed by our lab and well vetted in the scientific community and the literature. We will analyze particulate organic matter, dissolved organic matter and dissolved inorganic carbon from water specifically for this Sea Grant proposal. We will use fish from which we will excise muscle that are collected during NSF sampling for juvenile spotted seatrout otoliths. We will use isotope-ratio mass spectrometry to quantify C and N stable isotopes from these samples. The use of stable isotopes has burgeoned in the past decade, especially to clarify and quantify trophic interactions. Therefore we propose to analyze the d13C, d15N and C/N ratio of the dissolved organic matter (DOM) and the particulate organic matter (POM) that serve as the as the biogeochemical base of the seagrass beds. The C/N ratios will provide a qualitative measure of algal vs. terrestrial carbon within the seagrass beds, and the isotopic compositions will provide a first-order, integrated biogeochemical “tag” of nature organic matter in the water column that is reflected in the isotopic compositions of juvenile fish.  The d13C of dissolved inorganic carbon (DIC) will be determined so as to normalize our DOM and POM carbon isotopic results. 

There have been very few studies that have quantitatively linked the growth and survival of fish to their nursery habitats, even though federal and state laws require protection of “essential fish habitat”. The protection of habitat has received increased attention in Virginia, and nationwide with the promulgation of marine-protected areas and sanctuaries. However, it is very difficult to demonstrate direct value in preserving a seagrass bed to the ultimate production of fisheries resources. Clearly, we can not do that in one two-year grant, but we have been developing cutting-edge techniques and designing innovative research to move toward this long-term goal. In25-Jan-2008efining a demonstrable linkage though this model-system approach by weight of evidence. Although other fish and habitat may not have the tight coupling of our model system, our results are transferable nonetheless, even if less demonstrable in other systems. The results of our research can be used to show the importance of seagrass restoration in Chesapeake Bay in a way that will be clear to its citizens. Fewer beds, fewer fish. Moreover, our results from prior funding show dramatic changes in fish growth when runoff is reduced. With more research and further evidence, we may be able to demonstrate the value that conservation buffers would have on seagrass nurseries. Altogether, our results have relevance to Bay restoration and management of sub-aqueous habitats.