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Long-term dynamics and productivity of a successful invader: The first three decades of the bivalve Ensis directus in the western Wadden Sea
Dekker, R.; Beukema, J.J. (2012). Long-term dynamics and productivity of a successful invader: The first three decades of the bivalve Ensis directus in the western Wadden Sea. J. Sea Res. 71: 31-40. dx.doi.org/10.1016/j.seares.2012.04.004
In: Journal of Sea Research. Elsevier/Netherlands Institute for Sea Research: Amsterdam; Den Burg. ISSN 1385-1101, more
Peer reviewed article  

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Author keywords
    Long-term Data; Biomass; Secondary Production; Annual Recruitment; TidalFlats

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Abstract
    Results are reported of a long-term and multi-station monitoring study in the western Wadden Sea on numerical densities, biomass, individual weights, and production of populations of the newly (late 1970s) introduced bivalve Ensis directus. Data are available on 18 fixed sites, sampled twice-annually ever since 1982 (13 truly intertidal stations and 2 in the transition zone to the subtidal) or 1989 (3 subtidal stations). After a slow start in the 1980s, numbers and biomass rapidly increased during the last 2 decades to reach locally high values in recent years: annual averages of tens of g ash-free dry mass m(-2) in the subtidal and even of around 100 g AFDM m(-2) in the transition zone, where it became the dominant species as to biomass and annual production. Temporal and local variability in production (and resulting biomass) could largely be explained by variation in numerical densities of 0-group individuals at the end of the first winter of their life. Highest densities and growth rates were observed in the transition zone. In the true subtidal, recruitment and growth rates were on average lower. In the true intertidal, abundances remained invariably low, as a consequence of relatively low recruitment and in particular poor subsequent survival. Within the "optimal" transition zone along tidal channels, the benthic habitat is highly dynamic, as it is exposed to physical disturbance resulting from both wave action and strong tidal currents. E. directus appears to be one of the few large-bodied benthic invertebrate species that can cope with the unstable sands in this zone. Most of its exceptional success may be explained by the occupation of this probably under-utilized habitat in Europe.

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