Evidence for spatial variability in estuarine food webs

The importance of phytoplankton, benthic microalgae, fresh and salt marsh emergent plants, and terrestrial organic matter to the estuarine food w e b of Plum Island Sound, Massachusetts, USA, was evaluated using the stable isotopes of carbon, nitrogen and sulfur. Three distinct regions of the estuary were examined: the oligohaline upper estuary with high freshwater inputs and fringing fresh marsh, the middle estuary with extensive salt marsh, and the lower estuary with a greater proportion of open bay and a direct connection to open ocean. In each region, sources of organic matter for pelaglc and benthic aquatlc consumers were compared with the range of organic matter sources available in the entire estuary. Consumers in all regions relied most heavily on locally produced organic matter. In the upper estuary, most consumers had 613C values of -29 to -2 1 ' L and 634S values of about 8?L, whlch indicated dependence on a mixture of fresh marsh emergent vegetation and phytoplankton from the oligohaline region. In the middle and lower estuary, consumers had 613C values of -23 to -1 5 ' k and 634S values of 5 to 154., resembling a mixture of Spal-tina spp., benthic m~croalgae and marine phytoplankton. Terrestrial organic matter was of minimal importance in the upper estuary and was not evident in the food webs of the middle and lower estuary. The isotopic values of pelagic consumers reflected a greater dependence on phytoplankton than those of benthic consumers, which were closer to fresh or salt marsh emergent vegetation and benthic microalgae. Several of the patterns revealed by examination of organic matter sources for the food w e b of the Plum Island Sound estuary have implications for our general understanding of estuarine food webs. First, while there is substantial spatial heterogeneity in organic matter sources within a single estuary, consumers tend to utilize sources of organic matter produced In the same region of the estuary in which they reside. Second, consumer dependence on terrestrially derived riverine organic matter is minimal, even in the relatively small region of the estuary where ~t s availability is the highest Third, benthic and p e l a g ~c organisms rely on different mixes of organic matter sources.