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In November 1983, the construction of the first part of an assembling plant of jackets to support oil-drilling platforms began at Punta Banda estuary, a 16.40 km2 coastal estuary located 150 km south of the USA-México border. The area that is being built on (0.45 km2) is located at the south-west corner of the estuary and is delimited by a dike. The second part of the construction, covering a further 0.40 km2, will extend to the south-east and will require some deflection of the main circulation channel.

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In the absence of chemical or physical gradients, random displacement of organisms can result in unpredictable distribution patterns. In spite of a limited locomotive capability, marine nematodes may choose where to settle after re-suspension and may maintain their position in the sediment under calm conditions, leading to small-scale (,1 m) spatial variability. However, in more energetic environments, nematodes become re-suspended with sediments and re-distributed at distances dependent on prevalent hydrodynamic regimes, from metre-to decametre-scale or more. In this study, we tested the hypothesis that micro-habitats (i.e. runnels and sandbars) in a macrotidal sandy beach influence the distribution patterns of freeliving marine nematodes by exhibiting contrasting hydrodynamic regimes. Specifically, we predicted patchier distributions in the calmer environment (runnel). We sampled nematodes in each habitat from ,1 m to decametre scales. Our results show more heterogeneous spatial distributions in the runnel, presumably owing to a predominance of active displacement under calmer conditions and sediment cohesion by algal films. Biological similarity among runnel replicates was low, whereas replicates from the sandbar exhibited higher similarity, presumably because of homogenization of the sediment and inhabiting fauna by tidal currents. A significant negative correlation between biological similarity and sampling distance was found in the runnel, but not in the sandbar. The most similar samples were the closest in the runnel and the most distant in the sandbar. More patchily distributed taxa were found in the runnel and a larger fraction of homogeneously or randomly distributed taxa in the sandbar. We conclude that different hydrodynamic regimes in contrasting intertidal micro-habitats significantly influenced the nematofaunal distribution, resulting in different spatial patterns next to one another in the same beach. This has significant implications for sampling and monitoring designs and begs the need for detailed studies about the physical and biological processes governing meiobenthic communities.

Changes in salt marsh vegetation were analyzed over a two-year period (November 1984-November 1986 following the construction of a dike in the southwest corner of Punta Banda Estuary, Baja California, Mrxico.Changes included: increased interstitial soil salinity, reduced soil moisture, increased mortality of Spartina foliosa and decreased middle marsh species diversity due to the elimination of annual and short-lived species.The sea-side edge of the middle marsh shifted its boundary to a lower elevation. By the end of 1986, dominant species were Salicornia virginica, Batis maritima and Frankenia grandifolia. By October 1988, only a few patches of S. foliosa persisted at the water edge, and it appeared that the community was not yet stable. The potential future of the estuary is evaluated.

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