Changes in land use and the development of coastal regions around the world have markedly increased rates of sediment input into estuarine and coastal habitats. Field studies looking at the consequences of terrestrial sediment deposition, water‐borne sediment, and long‐term changes in habitats indicate that increasing rates of sediment loading adversely affect the biodiversity and ecological value of estuarine and coastal ecosystems. Managing this threat requires means with which to convey the magnitude of the problem, forecast long‐term trends, and assess the risks associated with changes in land use. Here we focus on approaches for assessing the risks of changes in land use, which include combining biological effect studies with catchment and hydrodynamic modeling, using statistical models that forecast the distribution and abundance of species relative to changes in habitat type, and using sensitive species that play important ecological roles as indicators of change.
Studies of the influence of small-scale disturbances on soft-bottom communities have usually been conducted on one type of community only. We utilized polychaete and bivalve dominated macrobenthic communities in 2 physically similar intertidal sandflat sites to study differences in recolonization of pits created by feeding eagle rays Myljobatis tenuicaudatus. In both communities ray pits were sampled on 6 occasions until 12 d after their creation. Ray pits rapidly infilled with sediment of similar grain size to that of the surrounding sandflat. Organic carbon content only became elevated in the pits in the polychaete dominated community. The intensity of ray disturbance ind~cated that sediment within an area of 700 to 800 mZ at either site would be turned over about every 70 d. Rapid recolonization of pits by macrofauna occurred in both communities, although bivalves tended to recolonize more rapidly than polychaetes. An epibenthic crustacean was the only species to indicate possible preferential exploitation of pits. Rapid recolonization and sediment infilling emphasise the importance of passive transport of adults into pits. The tube-mat forming polychaete Boccardia syrtis, dominant at one site, was the only common species which did not colonize pits in the same proportion to that found in the sediment adjacent to pits. The rate of sediment turnover by rays and the rapid recolonization by macrofauna indicate that rays may tend to smooth out distribution patterns, particularly those of long-lived infauna, and play a role in maintaining dominance patterns in both polychaete and bivalve communities.
An experiment was designed to assess the role of 2 different predators in determining the macrobenthic community structure of an intertidal sandflat. The 2 predators were: shorebirds which feed throughout the year by removing individual prey items, and eagle rays Myliobatis tenuicaudatus which are only present during the summer and disturb large volumes of sediment when extracting prey. The experiment consisted of bird exclusion, ray + bird exclusion and reference plots. Samples were collected from each plot on 2 occasions: 6 mo after the initiation of the experiment, when rays were absent and common bivalve densities were high following recruitment, and 8 mo later when rays were present and bivalve population structure was not dominated by new recruits. At the end of the experiment analysis of surficial sediment features did not indicate the experiment was confounded by localised modifications of sediment or hydrodynamic conditions. Community level differences on both occasions were driven by effects on common taxa. The seasonality of effects in our experiment precluded direct comparison of the 2 predators. However, the 6 mo results indicated that bird predation resulted in indirect effects due to adult/juvenile interactions amongst the dominant bivalve Macomona liliana. At the end of the experiment, 14 mo after its initiation, analysis of common taxa generally revealed direct negative effects of predation, with significantly high densities in the ray + bird exclusion treatment. Infaunal density changes in response to the exclusion of shorebirds and rays did not indicate the presence of multiple trophic levels in this infaunal assemblage. Differences between the results obtained from the bird exclusion and the ray + bird exclusion treatments on the first sampling occasion were attributed to an edge effect around the bird exclusion plots which effectively increased their area. This edge effect emphasises the importance of infaunal mobility and its potential to swamp predator effects. The results of this experiment highlight the importance of considering the role of predators within an appropriate spatial and temporal context.
Spatial autocorrelation correlograms based on Moran's co- collected from intertidal sandflats of Manukau Harbour (New Zealand) during October, 1987. Patterns of heterogeneity on a scale smaller than inter-sample distance, homogeneous density patches (5 to 30 m radius) and gradients in abundance running through sample sites (9 000 m 2) were identified. Patterns could be defined even for species with distributions which, based on the variance:mean ratio test, were not significantly different from random. The possible influence on two of the study sites of sediment disturbances generated by feeding rays is discussed. Identification of spatial patterns is considered an important aspect of the design of surveys and manipulative field experiments.
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