A number of recent studies have shown that water-column processes exert an important influence on meiofaunal recruitment and colonization of new areas. T h~s paper reviews those studies which have investigated the water-column occurrence of meiofauna and the subsequent settlement process. Two distinct patterns exist for recruitment via water-column pathways: active entry of meiofauna into the water and passive erosion of meiofauna from the sediments. A conceptual model is proposed in which 4 factors interact to determine whether active vs passive mechanisms are most important for a given community: taxonomic con~position, hydrodynamics, aboveground structure, and disturbance. For the melofauna of areas which are hydrodynamically benign and dominated by actice swimmers (e. g, seagrass beds), water-column recruitment should involve substrate cholce through active swimming. In areas which are free of aboveground structure and more rigorous hydrodynamically (e. g. tidal flats, beaches) passive recruitment processes dominate and are modified by behaviors which may influence transport and settlement. In all habitats, structure probably acts to enhance active emergence to some extent while disturbance events may lead to increased suspension and possibly actlve emergence Future directions are discussed with an emphasis on the need for the development and standardization of new methodologies which can be used in a variety of habitats.

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Recent studies have shown that meiofauna frequently occur in the water column and may be highly mobile due to suspension by water currents or any process that disturbs the sediments. We hypothesized that tidal variability in meiofauna sediment densities and distributions may be significant. As a test of this hypothesis, replicate sediment cores were collected in intertidal, slope (creekbank just below water level), and subtidal muds to determine if densities of benthic copepods varied over a tidal cycle. As determined by analysis of variance, the number of copepods within each area clearly varied with the stage of the tide. In the intertidal and subtidal areas there were significantly more copepods in the sediment at both slack high and low tide than at flooding or ebbing tides. This pattern may be due to resuspension of meiofaunal copepods from the sediment into the water column via tidal current action. At the slope site such a tidal response was more complex and observations on the distribution of certain species in the 3 areas suggest that movement of animals occurred between intertidal and slope areas. All intertidal species did not display the same tidal response, and in the slope and subtidal areas all developmental stages (adults, copepodites, nauplii) did not display the same tidal response. These differences were likely due to differences in the size and/or behavior of adults vs juveniles. The processes which likely act to redistribute mud-meiofauna in many systems have hitherto been largely ignored, yet may have important consequences for the communities.

We investigated the influence of the bottom-feedmg fish Leiostomus xanthurus Lacepede on the vertical microdistribution of meiobenthos. Sectioned cores which were taken at low tide in the field did not exhibit evidence that fish, which had foraged 3 to 4 h earlier, affected the vertical distribution of meiofauna. However, field cores collected where fish were feeding did show reductions in meiofaunal abundances in the top 2 mm of sediment. Controlled flume experiments also showed that fish influenced the vertical distribution of meiofauna. For copepods, copepod nauplii and foraminlferans, reductions in abundances occurred in the top 4 mm of sediments due to fish consumption and/or migration into the water. For nematodes, reductions in the top 2mm occurred due to mortality (but not necessarily consumption by fish), as well as possible migration deeper into the sediments when fish were present. In conjunction with this study, we examined the possibility that estimates of total abundance of meiofauna may be influenced by the process of sectioning the cores. We found that total meiofauna abundance was significantly greater in sectioned than in non-sectioned cores, suggesting that investigators must test the effects of sample handling when choosing enumeration methods.

Secondary production of Microarthridion littorale, an estuarine meiobenthic copepod, was determined in a South Carolina (USA) salt marsh by the use of size-frequency distributions and age specific growth rates. For all individuals production was highest in 1976 during late summer, with a daily production rate of 1.2pg 10crn-~ d- '. In 1978, production for adults was much lower, less than half of the 1976 adult values. An annual production of 0.14g m -' dry weight is estimated, yielding a production/biomass ratio of 18.0 yr-' in 1976.