The estuarine fish community and food web structure in areas of sea lettuce (Ulva lactuca) and eelgrass (Zostera marina) within the Stanley River Estuary, Prince Edward Island

Agriculture is a major industry on Prince Edward Island (PEI), covering approximately 44% of the land area. Large quantities of nutrients from artificial fertilizers are entering surface and ground waters that feed estuaries and bays, replacing eelgrass (Zostera marina) with dense blooms of sea lettuce (Ulva lactuca) and resulting in anoxic events. This project aimed to determine whether the nearshore estuarine fish community and food web structure differed between areas dominated by sea lettuce and areas with mainly eelgrass. Monthly beach seining was conducted from April to August 2008 to sample the fish community at five stations in the Stanley River estuary on PEI, where habitat varied from primarily eelgrass beds at the downstream stations to complete sea lettuce cover at the upstream stations. The fish community differed significantly among stations and months. In general, stations with large quantities of sea lettuce had fewer Atlantic silverside (Menidia menidia) and more mummichog ( Fundulus heteroclitus) than those with less sea lettuce in April, more ninespine stickleback (Pungitius pungitius) and fewer northern pipefish (Syngnathus fuscus) in June, and more young-of-the-year (YOY) Gasterosteus spp. and YOY fourspine stickleback ( Apeltes quadracus), yet fewer northern pipefish, in July. In August, the main differences were fewer YOY Atlantic silverside and YOY northern pipefish and more YOY mummichog at the upstream sea lettuce stations. In addition, primary producer, invertebrate, and fish samples were collected at the same five stations in August 2007, as well as monthly from April to August in 2008, in order to assess temporal and spatial variability in the food web structure. The food web was described using carbon and nitrogen stable isotope analysis and stomach contents analysis was performed on two common fish species, mummichog and fourspine stickleback. Overall, stable isotope values did not differ between fish caught at sea lettuce and eelgrass stations. This indicated that there was no shift from a pelagic (lower delta13C values) to a benthic-driven (higher delta13C values) food web at stations with an abundance of sea lettuce and, presumably, associated benthic invertebrates. Gammarid amphipods were by far the most prevalent food item for mummichog and fourspine stickleback. Except in April, there were no significant differences in mummichog diet between sea lettuce and eelgrass stations. While increased sea lettuce alters the structural habitat of an estuary, and potentially increases the abundance of some invertebrates, this does not result in fish having a larger proportion of their diet corning from benthic resources.