No abstract

No abstract

The performance of the progeny of eastern oysters Crassostrea virginica from Louisiana selected for resistance to dermo, caused by Perkinsus marinus (referred to as 'OBOY') and of wild oysters collected from Louisiana (Calcasieu Lake) and Alabama (Cedar Point, Perdido Pass), USA, estuaries was compared for their potential use in aquaculture. Seed oysters from each stock were deployed in September 2011 at 2 dermo-endemic sites, Dauphin Island and Sandy Bay, Alabama, using an adjustable longline system, and their survival and shell heights were monitored bimonthly. P. marinus infection intensity and condition index were measured at deployment and in March, July and September 2012. The OBOY stock showed lower mortality than the unselected stocks (Cedar Point, Perdido Pass, Calcasieu Lake) at Dauphin Island, and both Louisiana stocks had lower mortality than the Alabama stocks at Sandy Bay, a slightly more saline site. Mortality increased in summer, especially between July and September, concomitant with increasing P. marinus infection intensities at the higher temperatures and favorable salinities. At the higher salinity site, both Louisiana stocks had lower P. marinus infection intensities than the Perdido Pass stock, the stock with the highest percentage of oysters with moderate and heavy infection and cumulative mortality. The OBOY stock reached greater mean shell height than Calcasieu Lake and Perdido Pass stocks. Condition index of the oyster stocks decreased by more than half between March and July following expected spawning. Differences in stock performance highlight the importance of stock selection for aquaculture in dermo-endemic estuaries of the northern Gulf of Mexico.

We assessed the individual and combined effects of removing large predators and enriching water column nutrients on shoalgrass Halodule wrightii meadows in Big Lagoon, Florida, USA. To simulate the first-order effects of large predator reductions, we stocked 2.0 m 2 enclosures with elevated (~3 to 4× ambient) densities of the omnivorous pinfish Lagodon rhomboides, the dominant fish in local seagrass habitats, and we supplemented N and P in the water column to nearly 3× ambient levels. Monthly determinations of water column nutrients and chlorophyll a (chl a), coupled with bimonthly measurements of leaf epiphyte biomass, seagrass growth and biomass, and beginning and ending comparisons of mesograzer abundance, were used to evaluate the effects of increasing nutrient supply and changing food web structure. Results showed significant predator and nutrient effects, although there were fewer consumer effects and more negative nutrient effects on seagrasses than in our previous experiments, which had shown that mesograzers ameliorated the harmful effects of elevated nutrients on seagrasses. Epiphyte proliferation in enrichment treatments did not occur; thus, algal overgrowth could not explain the negative effects of nutrient loading on seagrass biomass. Instead, nutrient loading resulted in nitrogen-rich shoalgrass, and it appears that this high-quality food stimulated pinfish herbivory. Elevated pinfish consumption of the enriched shoalgrass then resulted in the decline of seagrass biomass in enrichment enclosures. These results add additional complexity to understanding and predicting the effects of eutrophication in coastal waters.

This study, conducted in 1997, reports the first estimates of the impacts of the proliferation of an exotic submerged aquatic vegetation (SAV) species (Myriophyllum spicatum) on macroinvertebrate production via comparisons with two co-occurring native SAV species (Heteranthera dubia and Vallisneria americana) in the tide-influenced Mobile-Tensaw Delta (located in the northcentral Gulf of Mexico, 30°40′ N, 87°55′ W). Production of macroinvertebrates was greatest on M. spicatum and H. dubia and least on V. americana. The key determinant of these differences was a greater abundance of amphipods (Gammarus mucronatus) found on the leaves of M. spicatum and H. dubia. Macroinvertebrate production on M. spicatum was three times greater (>1 kg m −2 year −1 ) than on either of the native SAV species. No-choice palatability tests showed that these differences could not be attributed to differences in invertebrate grazing on these plants. Instead, it is probable that the high production within the structurally complex M. spicatum and H. dubia was the result of reduced predator foraging efficiency. If true, then the presence of this exotic species probably renders this elevated production inaccessible to most highorder predators.