Chronic Exposure of Corals to Fine Sediments: Lethal and Sub-Lethal Impacts

Understanding the sedimentation and turbidity thresholds for corals is critical in assessing the potential impacts of dredging projects in tropical marine systems. In this study, we exposed two species of coral sampled from offshore locations to six levels of total suspended solids (TSS) for 16 weeks in the laboratory, including a 4 week recovery period. Dose-response relationships were developed to quantify the lethal and sub-lethal thresholds of sedimentation and turbidity for the corals. The sediment treatments affected the horizontal foliaceous species (Montipora aequituberculata) more than the upright branching species (Acropora millepora). The lowest sediment treatments that caused full colony mortality were 30 mg l−1 TSS (25 mg cm−2 day−1) for M. aequituberculata and 100 mg l−1 TSS (83 mg cm−2 day−1) for A. millepora after 12 weeks. Coral mortality generally took longer than 4 weeks and was closely related to sediment accumulation on the surface of the corals. While measurements of damage to photosystem II in the symbionts and reductions in lipid content and growth indicated sub-lethal responses in surviving corals, the most reliable predictor of coral mortality in this experiment was long-term sediment accumulation on coral tissue

Characteristics of Carbonates of Gorgonian Axes (Coelenterata, Octocorallia)

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Day-night activity rhythmn of the cold seep shrimp Alvinocaris stactophila (Caridea: Alvionocarididae) from the Gulf of Mexico

The activity rhythm of the cold seep shrimp Alvinocaris stactophila from the Gulf of Mexico slope (650 m depth) was investigated in the laboratory in relation to an artificial 12 h light-dark regime. Animals were sampled with a submersible and transferred into individual aquaria where their activity was monitored for 5 d by taking digital video snapshots every 30 s. An observer analysed the footage by counting the number of times an animal crossed two perpendicular lines drawn on a PC screen per 30 min interval. Resulting time series were represented over consecutive days and the waveform analysis was used to precisely assess the phase (i.e. the peak timing) and its limits (i.e. onset and offset) in relation to light ON and OFF. The majority (73%) of animals showed a marked nocturnal pattern of activity with number of movements close to zero during the photophase. Waveform analysis showed that the behavioural transition from activity to inactivity after light ON occurred within approximately 1 h. Considering the general lack of knowledge on the regulation of activity rhythms of crustaceans from subtidal areas, the present data provide a new insight on the role played by light in the regulation of animal activity rhythm in deep-water environments such as those of the cold seeps in the Gulf of Mexico.<br/

Compositional Effects of Sea-Level Rise in a Patchy Landscape: The Dynamics of Tree Islands in the Southeastern Coastal Everglades

The landscape structure of emergent wetlands in undeveloped portions of the southeastern coastal Everglades is comprised of two distinct components: scattered forest fragments, or tree islands, surrounded by a low matrix of marsh or shrub-dominated vegetation. Changes in the matrix, including the inland transgression of salt-tolerant mangroves and the recession of sawgrass marshes, have been attributed to the combination of sea level rise and reductions in fresh water supply. In this study we examined concurrent changes in the composition of the region’s tree islands over a period of almost three decades. No trend in species composition toward more salt-tolerant trees was observed anywhere, but species characteristic of freshwater swamps increased in forests in which fresh water supply was augmented. Tree islands in the coastal Everglades appear to be buffered from some of the short term effects of salt water intrusion, due to their ability to build soils above the surface of the surrounding wetlands, thus maintaining mesophytic conditions. However, the apparent resistance of tree islands to changes associated with sea level rise is likely to be a temporary stage, as continued salt water intrusion will eventually overwhelm the forests’ capacity to maintain fresh water in the rooting zone

Alternate unstable states: Convergent paths of succession in hydrocarbon-seep tubeworm-associated communities

Previous studies have shown clear, predictable successional trends in habitat characteristics and community structure in tubeworm aggregations at 3 similar hydrocarbon-seep sites on the central upper Louisiana slope of the Gulf of Mexico. In this study, we examine these trends in quantitative community collections from 7 additional hydrocarbon-seep sites widely distributed in the northern Gulf of Mexico. The relative proportions and sizes of Lamellibrachia luymesi and Seepiophila jonesi in tubeworm aggregations were similar at new and central sites, though S. jonesi dominated some collections from new sites, a situation not previously observed at central sites. In general, sulfide declined with increasing aggregation age (average size of tubeworms), but there was more variability in this trend at the new sites. Tubeworm-associated community composition was similar at new and central sites, with only a few rare species collected at the new sites for the first time. The most significant differences in the communities at new sites were the lower relative abundance of various gastropod species, and the absence of gastropods from collections made at the Viosca Knoll site. This community type was largely restricted to young aggregations at new sites that were more isolated from other tubeworm aggregations and consisted of higher proportions of S. jonesi. As succession proceeds from young to old aggregations, many of the previously described processes were apparent at the new sites including a reduction in biomass and a shift in trophic structure from endemic primary consumers to non-endemic higher order predators. Regardless of community composition in young aggregations, succession converges on similar late-stage community types

Alvin Explores the Deep Northern Gulf of Mexico Slope

Many of the world's productive deepwater hydrocarbon basins experience significant and ongoing vertical migration of fluids and gases to the modern seafloor. These products, which are composed of hydrocarbon gases, crude oil, formation fluids, and fluidized sediment, dramatically change the geologic character of the ocean floor, and they create sites where chemosynthetic communities supported by sulfide and hydrocarbons flourish. Unique fauna inhabit these sites, and the chemosynthetic primary production results in communities with biomass much greater than that of the surrounding seafloor