The effects of intensive aquaculture on nutrient residence time and transport in a coastal embayment

Aquaculture in many countries around the world has become the biggest source of seafood for human consumption. While it alleviates the pressure on wild capture fisheries, the long-term impacts of large-scale, intensive aquaculture on natural coastal systems need to be better understood. In particular, aquaculture may alter habitat and exceed the carrying capacity of coastal marine ecosystems. In this paper, we develop a high-resolution numerical model for Sanggou Bay, one of the largest kelp and shellfish aquaculture sites in Northern China, to investigate the effects of aquaculture on nutrient transport and residence time in the bay. Drag from aquaculture is parameterized for surface infrastructure, kelp canopies, and bivalve cages. A model for dissolved inorganic nitrogen (DIN) includes transport, vertical turbulent mixing, sediment and bivalve sources, and a sink due to kelp uptake. Test cases show that, due to drag from the dense aquaculture and thus a reduction of horizontal transport, kelp production is limited because DIN from the Yellow Sea is consumed before reaching the interior of the kelp farms. Aquaculture drag also causes an increase in the nutrient residence time from an average of 5 to 10 days in the middle of Sanggou Bay, and from 25 to 40 days in the shallow inner bay. Low exchange rates and a lack of DIN uptake by kelp make these regions more susceptible to phytoplankton blooms due to high nutrient retention. The risk is further increased when DIN concentrations rise due to river inflows.     

Spatio-temporal variability of polychaete colonization at volcanic CO2 vents indicates high tolerance to ocean acidification

Ocean acidification is predicted to have negative effects on marine biota, resulting in the loss of biodiversity and changes in marine ecosystem structure and function. However, some species and life stages may be capable of thriving in low pH conditions, either due to their natural ability to tolerate stressful low pH–high pCO₂ conditions and/or alteration of species interactions caused by changes in pH profiles, or due to evolutionary trade-offs. A better understanding of which species may be capable of tolerating ocean acidification can guide future research into the mechanisms for physiological and ecological resilience to future carbon dioxide (CO₂) conditions. We investigated the colonization of selected polychaete species along a pH gradient originating from shallow, coastal volcanic CO₂ vents (Ischia, Italy). Colonization was quantified by exposing artificial invertebrate collectors attached to the substratum for 30 days during different periods of the year (late spring, fall and late winter). Three species, Amphiglena mediterranea, Platynereis dumerilii and Syllis prolifera, were present and abundant along the gradient throughout the year. All three species were significantly more abundant in the most acidified areas, confirming their high tolerance and capacity to cope with very low pH. Abundances of all three species were compared to data previously collected via collectors suspended in the water column. More individuals were found in the collectors attached to the substratum, suggesting that abundances may have previously been underestimated. This is likely due to the close proximity of these collectors with the natural rocky substratum. All three species exhibited similar temporal variability, consistent with their life cycle and reproductive biology. Our results demonstrate high tolerance of the species for low and variable pH and corroborate their use as robust models to explore the capacity to cope with low pH–high pCO₂ conditions, both in the natural vent systems and in the laboratory.     

Changes in Genetic Structure of Posidonia oceanica at Monterosso al Mare (Ligurian Sea) and Its Resilience Over a Decade (1998–2009)

Genetic differences in the Posidonia oceanica meadow of Monterosso al Mare (NW Mediterranean, Marine Protected Areas (MPAs) "Cinque Terre") were compared in three stations, at an increasing distance from a source of impact (beach nourishment) in the recent decade. Random amplified polymorphic DNA analysis showed a higher genetic variability (>20?%) in the area directly subjected to the stress, increasing with time. Clone integration, confirmed by phenotypic analysis, showed increases both in shoot density and leaf length connected to genetic differences observed in DNA fingerprints of new shoots. Analysis of molecular variance (AMOVA) showed 45?% individual differences within populations and 54?% among the populations. The fixation index (F (ST)?=?0.54), of the genetic differentiation, showed a marked difference between the populations at different temporal scales. Over a decade AMOVA indicated genetic variations from 28?% (1998) to 54?% (2009). These results make it clear that in the P. oceanica population examined the environment had, in ten years, selected those clones which were more resistant to the anthropogenic impact, despite being subjected to the effects of the resuspension of fine sediments. These findings could help to explain both the survival of the regressed Mediterranean P. oceanica meadows in areas subjected to moderate impacts and the extreme variability in success of revegetation experiments. Management of the ecological disturbance here described indicates also the timescale in population response to stress and its increased resilience in MPAs.     

Reliance of mobile species on sensitive habitats: a case study of manta rays (Manta alfredi) and lagoons

Quantifying the ecological importance of individual habitats to highly mobile animals is challenging because patterns of habitat reliance for these taxa are complex and difficult to observe. We investigated the importance of lagoons to the manta ray, Manta alfredi, a wide-ranging and vulnerable species in a less-disturbed atoll ecosystem. Lagoons are highly sensitive to anthropogenic disturbance and are known to be ecologically important to a wide variety of mobile species. We used a novel combination of research tools to examine the reliance of M. alfredi on lagoon habitats. Stable isotope analysis was used to assay the recent energetic importance of lagoons to M. alfredi; high-resolution tracking data provided information about how M. alfredi utilised lagoonal habitats over long and short time periods; acoustic cameras logged patterns of animal entrances and departures from lagoons; and photo identification/laser photogrammetry provided some insight into why they may be using this habitat. M. alfredi showed strong evidence of energetic dependence on lagoon resources during the course of the study and spent long periods of residence within lagoons or frequently transited into them from elsewhere. While within lagoons, they demonstrated affinities for particular structural features within this habitat and showed evidence of temporal patterning in habitat utilization. This work sheds light on how and why M. alfredi uses lagoons and raises questions about how this use may be altered in disturbed settings. More generally, these observations contribute to our knowledge of how to assess the ecological importance of particular habitats situated within the broader home range of mobile consumers.     

Estuarine Vegetated Habitats as Corridors for Predator Movements

Abstract: The spatial proximity of one habitat to another can strongly influence population and community dynamics. We investigated whether the proximity of intertidal oyster reefs to vegetated estuarine habitats, salt marshes, and seagrass beds, affects the abundance and community structure of benthic macroinvertebrates on reefs and predator-prey interactions between mobile predators and bivalves living on reefs. Benthic macroinvertebrate abundance was highest on reefs spatially separated from salt marshes. Macroinvertebrate species richness was highest on reefs separated from both salt marshes and seagrass beds. Comparisons of predation on juvenile bivalves transplanted to reefs for 7–12 days indicated that survivorship of clams was greatest on reefs spatially separated from both salt marshes and seagrass beds, whereas reef proximity to vegetated habitats did not affect the survivorship of oysters. The foraging behavior of blue crabs may explain patterns of macroinvertebrate abundance and clam survivorship among reefs with different proximity to vegetated habitats. In experiments conducted in 30-m² field enclosures, blue crabs had higher predation rates on hard clams transplanted onto artificial reefs adjacent to salt marshes or seagrass beds than on reefs separated from both habitats by unvegetated sand bottom. Thus, vegetated habitats appeared to act as corridors by facilitating the access of blue crabs to oyster reefs and enhancing the intensity of blue crab predation. Such an understanding of the effects of landscape characteristics of estuarine habitats on their value as habitats for estuarine organisms can be used to predict the consequences of habitat fragmentation on ecosystem function and to improve strategies for habitat and species conservation and restoration.     

Acute effects of removing large fish from a near-pristine coral reef

Large animals are severely depleted in many ecosystems, yet we are only beginning to understand the ecological implications of their loss. To empirically measure the short-term effects of removing large animals from an ocean ecosystem, we used exclosures to remove large fish from a near-pristine coral reef at Palmyra Atoll, Central Pacific Ocean. We identified a range of effects that followed from the removal of these large fish. These effects were revealed within weeks of their removal. Removing large fish (1) altered the behavior of prey fish; (2) reduced rates of herbivory on certain species of reef algae; (3) had both direct positive (reduced mortality of coral recruits) and indirect negative (through reduced grazing pressure on competitive algae) impacts on recruiting corals; and (4) tended to decrease abundances of small mobile benthic invertebrates. Results of this kind help advance our understanding of the ecological importance of large animals in ecosystems.     

Evaluating stream restoration projects

River and stream restoration projects are increasingly numerous but rarely subjected to systematic postproject evaluation. Without conducting such evaluation and widely disseminating the results, lessons will not be learned from successes and failures, and the field of river restoration cannot advance. Postproject evaluation must be incorporated into the initial design of each project, with the choice of evaluation technique based directly upon the specific project goals against which performance will be evaluated. We emphasize measurement of geomorphic characteristics, as these constitute the physical framework supporting riparian and aquatic ecosystems. Techniques for evaluating other components are briefly discussed, especially as they relate to geomorphic variables. Where possible, geomorphic, hydrologic, and ecological variables should be measured along the same transects. In general, postproject monitoring should continue for at least a decade, with surveys conducted after each flood above a predetermined threshold. Project design should be preceded by a historical study documenting former channel conditions to provide insights into the processes suggest earlier, potentially stable channel configurations as possible design models.     

Feeding and burrowing ecology of two East African mangrove crabs

The behavior and ecology of two mangrove crabs,Sesarma meinerti De Man, 1887 andCardisoma carnifex (Herbst, 1794) were investigated at the beginning of the rainy season (October–November 1988) at Mida Creek, Kenya. Both species occupy upper intertidal levels, above mean high-water neap, and completely overlap in their zonation. Each burrow lasts ca. 3 wk, with no significant difference between the species. Both are more active around dusk and dawn and also follow a similar trend in their foraging activity, but differ in that burrowing is mainly diurnal inC. carnifex and nocturnal inS. meinerti. A hierarchy of food preference, established by offeringC. carnifex andS. meinerti leaves of five mangrove species, proved similar for both, withBruguiera gymnorhiza ranking first andAvicennia marina last, but significant only forC. carnifex. A rough estimate of the amount of litter consumed by these two species and of the soil mixed up by their burrowing activity indicates that they play a role of primary importance in the ecology of East African mangroves.