PHYSICAL–BIOLOGICAL COUPLING ON OYSTER REEFS: HOW HABITAT STRUCTURE INFLUENCES INDIVIDUAL PERFORMANCE

A large-scale field experiment was conducted to test whether the physical structure of biogenic reef habitat controls physical conditions (hydrodynamics and hydrographics) with subsequent influence on the performance (recruitment, growth, and survival) of a benthic suspension feeder. The experimental system consisted of restored subtidal oyster reefs inhabited by the eastern oyster Crassostrea virginica. To determine whether the size of reefs influences the flow environment and oyster performance, reefs of four heights-tall (2 m), short (1 m), dredged (0.6 m), and low (0.1 m)-were constructed at 3-m water depth in the Neuse River estuary, North Carolina, USA. To test whether oyster performance varies with water depth and hydrographic conditions, tall and short reefs were also constructed at 6-m water depth. Flow speed, sedimentation, temperature, salinity, dissolved oxygen, and the performance of oysters were measured as a function of reef height, position on reef, and water depth over a 10-mo period. Flow speed was found to increase on reefs with reef height and elevation on reefs. Rates of sediment deposition were seasonally highest where flow speed was lowest, at the bases of reefs, and seasonally decreased with increasing water depth. More than 90% of the surface area of low reefs was buried after only 16 mo of exposure in the estuary, indicating that reef height controls habitat quality (and quantity) indirectly through its effect on flow. Short reefs and the bases of tall reefs at 6-m depth were exposed to a total of 26 d of hypoxia/anoxia. No other reef treatment was exposed to >5 d of hypoxia. Physical conditions on experimental reefs had a profound influence on the performance of oysters as the flow environment alone explained 81% of variability in oyster growth and mortality. Recruitment of oysters over a 2-mo period was slightly higher on the front bases than the crests of reefs, but did not vary with reef height or water depth. After 10 mo, the shell growth and condition index of genetically similar, hatchery-raised oysters were greatest on the crests of tall and short reefs, where flow speed and quality of suspended food material were highest, and sediment deposition was lowest. Growth was greatest overall at the crests of tall reefs located at 6-m water depth where flow speed was high, and the numbers of days exposed to hypoxia/anoxia and variation in salinity were lowest. Total percentage mortality of oysters after 10 mo was greater on low reefs located at 3-m depth than on all other reef types and was greater on the bases than crests of tall, short, and dredged reefs. Predation by crabs and fishes accounted for 4-20% of total oyster mortality and showed no pattern across reef treatments. Results of this experiment indicate (1) that the physical structure and location of biogenic habitat controls local physical variables and (2) that, in turn, physical variables, especially flow speed, have a profound influence on the performance of a resident species. Realization that an ecological function of habitat is to indirectly control local population production through physical-biological coupling should improve our ability to conserve, restore, and manage habitat and associated species diversity. Better ecological engineering of restored oyster reef habitat is likely to improve fishery production and help maintain estuarine biodiversity

Conserving oyster reef habitat by switching from dredging and tonging to diver-harvesting

A major cause of the steep declines of American oyster (Crassostrea virginica) fisheries is the loss of oyster habitat through the use of dredges that have mined the reef substrata during a century of intense harvest. Experiments comparing the efficiency and habitat impacts of three alternative gears for harvesting oysters revealed differences among gear types that might be used to help improve the sustainability of commercial oyster fisheries. Hand harvesting by divers produced 25−32% more oysters per unit of time of fishing than traditional dredging and tonging, although the dive operation required two fishermen, rather than one. Per capita returns for dive operations may nonetheless be competitive with returns for other gears even in the short term if one person culling on deck can serve two or three divers. Dredging reduced the height of reef habitat by 34%, significantly more than the 23% reduction caused by tonging, both of which were greater than the 6% reduction induced by diver hand-harvesting. Thus, conservation of the essential habitat and sustainability of the subtidal oyster fishery can be enhanced by switching to diver hand-harvesting. Management schemes must intervene to drive the change in harvest methods because fishermen will face relatively high costs in making the switch and will not necessarily realize the long-term ecological benefits

Assessing Withering Syndrome Resistance in California Black Abalone: Implications for Conservation and Restoration

Our overall research objectives were to (1) assess population trends along San Nicolas Island and in Monterey County; (2) optimize black abalone spawning methods; (3) develop and validate a real-time PCR assay for quantification of RLP loads (infection intensity); and (4) examine if progeny of surviving black abalone along the California islands are more resistant to WS than are animals without this disease pressure. At UCSB we were focused primarily on objective (2). We tested the following hypotheses to fulfill our objectives.Hypothesis 1: Black abalone spawning requires environmental conditions similar to their intertidal and shallow subtidal habitat, and not standard methods that were developed for subtidal species. Hypothesis 2: Quantitative real-time PCR can be used to quantify loads of the WS rickettsial bacterium (infection intensity) in abalone. Hypothesis 3: Juvenile black abalone recruiting along the California Channel Islands are more resistant to WS than are black abalone in northern Central California that have not experienced high disease (WS) selection pressure

HOW HABITAT DEGRADATION THROUGH FISHERY DISTURBANCE ENHANCES IMPACTS OF HYPOXIA ON OYSTER REEFS

Oysters are ecosystem engineers that create biogenic reef habitat important to estuarine biodiversity, benthic-pelagic coupling, and fishery production. Prevailing explanations for the dramatic decline of eastern oysters (Crassostrea virginica) during the last century overlook ecosystem complexity by ignoring interactions among multiple environmental disturbances. To explain oyster loss, we tested whether (1) mortality of oysters on natural oyster reefs varies with water depth (3 m vs. 6 m), (2) harvesting by oyster dredges reduces the height of oyster reefs, and (3) bottom-water hypoxia/anoxia and reduction in reef height through fishery disturbance interact to enhance mortality of oysters in the Neuse River estuary, North Carolina, USA. The percentage of oysters found dead (mean ± 1 SD) during a survey of natural reefs in May 1993 was significantly greater at 6-m (92 ± 10%) than at 3-m (28 ± 9%) water depth. Less than one scason's worth of oyster dredging reduced the height of restored oyster reefs by ∼30%. During stratification of the water column in summer, oxygen depletion near the seafloor at 6 m caused mass mortality of oysters, other invertebrates, and fishes on short, deep experimental reefs, while oysters and other reef associates elevated into the surface layer by sufficient reef height or by location in shallow water survived. Highly mobile blue crabs (Callinectes sapidus) abandoned burrows located in hypoxic/anoxic bottom waters but remained alive in shallow water. Our results indicate that interaction of reef habitat degradation (height reduction) through fishery disturbance and extended bottom-water hypoxia/anoxia caused the pattern of oyster mortality observed on natural reefs and influences the abundance and distribution of fish and invertebrate species that utilize this temperate reef habitat. Interactions among environmental disturbances imply a need for the integrative approaches of ecosystem management to restore and sustain estuarine habitat

Synthesis of linkages between benthic and fish communities as a key to protecting essential fish habitat

Several essential fish habitats lack the protections necessary to prevent degradation because of failure to integrate the scientific disciplines required to understand the causes of the degradation and failure to integrate the fragmented state and federal management authorities that each hold only a piece of the solution. Improved protection of essential habitat for demersal fishes requires much better synthesis of benthic ecology, fisheries oceanography, and traditional fisheries biology. Three examples of degraded habitat for demersal fishes and shellfishes are high-energy intertidal beaches, subtidal oyster reefs, and estuarine soft bottoms. In each case, both scientific understanding of and management response to the problem require a holistic approach. Intertidal beach habitat for surf fishes could be protected by constraints on the character of sediments used in beach nourishment and restriction of nourishment activity to biologically inactive seasons. Subtidal oyster-reef habitat for numerous crabs, shrimps, and finfishes could be protected and restored by reduction of nitrogen loading to the estuary and elimination of dredge damage to reefs. Estuarine soft-bottom habitat for demersal fin- and shellfishes could also be protected by reduction of the nutrient loading of the estuary, which could prevent associated problems of nuisance blooms and low dissolved oxygen. Although a broad general understanding of the nature of habitat degradation exists for each of these three examples, the interdisciplinary science needed to sort out the separate and interactive contributions of all major contributing factors is incomplete. Adopting the holistic approach embodied in the principles of ecosystem management sets a course for addressing both the scientific inadequacies and the management inaction

PREDATION STRUCTURES COMMUNITIES AT DEEP-SEA HYDROTHERMAL VENTS

The structure and dynamics of natural communities result from the interplay of abiotic and biotic factors. We used manipulative field experiments to determine the relative roles of abiotic conditions and biotic interactions in structuring deep-sea (2500 m depth) communities along environmental gradients around hydrothermal vents of the eastern tropical Pacific Ocean (East Pacific Rise, at 9°50' N). We tested (1) whether predation by crabs and fishes affects the recruitment of benthic species and subsequent community structure and (2) whether the effects of predation vary along the steep gradients of temperature, oxygen, sulfide, and metal concentrations near vents. Recruitment substrates (basalt cubic blocks, roughly 10 cm on a side), both uncaged and caged to exclude predators (crabs, fishes, whelks, and octopi), were deployed along a decreasing vent fluid-flux gradient. The exclusion of predators for 8 mo increased the abundance of small mobile gastropods and amphipod crustaceans but decreased the abundance of sessile invertebrates, including juvenile vestimentiferan worms, tubiculous polychaetes, and mussels. Effects of predation were strongest nearest to hydrothermal vents, where abiotic environmental conditions were most extreme but productivity and the overall abundances of benthic invertebrates and mobile predators were the greatest. Additional 5-mo experiments conducted at three different locations showed similar trends at all sites, indicating that these effects of predation on benthic community structure are repeatable. Stomach-content analyses of the most abundant predators found at vents indicated that the zoarcid fish (Thermarces cerberus) primarily feeds on the vent snail Cyathermia naticoides, the limpet Lepetodrilus elevatus, and the amphipod crustacean Ventiella sulfuris, the very species that showed the greatest increase following predator exclusion. In contrast, brachyuran (Bythograea thermydron) and galatheid (Munidopsis subsquamosa) crab stomachs did not contain small mobile grazers, and crabs presented with arrays of the most common vent invertebrate species preferred mussels and vestimentiferans over limpets. Our results indicate that predation by large mobile predators influences the structure of hydrothermal vent communities, directly by reducing the abundance of gastropod prey species, and indirectly by reducing gastropod grazing and by bulldozing of recruits of sessile invertebrates

High spatial variability in coral bleaching around Moorea (French Polynesia): patterns across locations and water depths

Mass coral bleaching events are one of the main threats to coral reefs. A severe bleaching event impacted Moorea, French Polynesia, between March and July 2002, causing 55+/-14% of colonies to suffer bleaching around the island. However, bleaching varied significantly across coral genera, locations, and as a function of water depth, with a bleaching level as high as 72% at some stations. Corals in deeper water bleached at a higher rate than those in shallow water, and the north coast was more impacted than the west coast. The relatively small scale of variability in bleaching responses probably resulted from the interaction between extrinsic factors, including hydrodynamic condition, and intrinsic factors, such as differential adaptation of the coral/algal association

Montana Kaimin, March 5, 1981

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