Population structure, dynamics and production of Hydrobia ulvae (Pennant) (Mollusca: Prosobranchia) along an eutrophication gradient in the Mondego estuary (Portugal)

Abstract

-Eutrophication in the Mondego estuary gave rise to qualitative changes in the benthic community, involving the replacement of eelgrass, Zostera noltii, by green algae such as Enteromorpha spp. and Ulva sp. It seems reasonable to assume that, through time, such changes may determine a selected new trophic structure. Hydrobia ulvae, a dominant species in terms of abundance and biomass, was studied with regard to life history, population dynamics and productivity in relation to changing environmental conditions along the eutrophication gradient. The purpose was to examine to what extent this species may adapt to the new emergent conditions. During the study period, H. ulvae population exhibited both temporal and spatial density variations. The settlement pattern did not change along the eutrophication gradient, and took place in March, June, July and September. However, the population was denser in the less eutrophied areas, corresponding to Z. noltii meadows, when compared to the eutrophied ones, where Enteromorpha spp. blooms are usually observed. Growth was continuous through life but growth rates were higher during spring, decreased from early summer to mid fall and practically ceased during winter, and then gradually increased again up to spring. Life span was estimated as 21 ± 3 months. Growth productivity (P) was estimated as 93.7 g AFDW⋅m ⋅year -1 in the strongly eutrophied area. The average annual biomass (B √ ) (standing stock) of the population was estimated as 70.2 g AFDW⋅m -2 in the Z. noltii meadows, 5.5 g AFDW⋅m -2 in the eutrophied area, and 7.4 g AFDW⋅m -2 in the strongly eutrophied area. P/B √ and E/B √ ratios were estimated as 1.3 and 4.8 in the Z. noltii meadows, 2.8 and 9.5 in the eutrophied area, and 4.5 and 13.2 in the strongly eutrophied area, respectively. As a pattern, the standing stock decreased as a function of increasing eutrophication, while P/B √ and E/B √ ratios increased following the same gradient. There is evidence that H. ulvae population structure and annual production are seriously affected by eutrophication, namely by macroalgal bloom dynamics. Moreover, results suggest that H. ulvae might be suffering a change in its adaptive strategy along the eutrophication gradient, becoming closer to a typical 'r' strategist in the strongly eutrophied areas. © 1999 Éditions scientifiques et médicales Elsevier SAS Eutrophication / Hydrobia ulvae / life cycle / population dynamics / productivity

Mass balanced models of the food web in three areas along a gradient of eutrophication symptoms in the south arm of the Mondego estuary (Portugal)

“…Macroalgal blooms of Ulva spp. have regularly been observed in the Mondego over the last 20 years (Flindt et al, 1997;Marques et al, 1997Marques et al, , 2003Lillebø et al, 1999;Pardal et al, 2000Pardal et al, , 2004Martins et al, 2001;Dolbeth et al, 2003;Cardoso et al, 2004). Nevertheless, such macroalgal blooms may not occur in exceptionally rainy years.…”

“…Sampling was conducted in three areas in the southern arm of the Mondego estuary that represent different environmental situations along a spatial gradient of eutrophication Lillebø et al, 1999;Pardal et al, 2000Pardal et al, , 2004Dolbeth et al, 2003;Cardoso et al, 2004;Neto, 2004) (Fig. 1): (a) a non-eutrophic area (Zostera noltii Hornem beds), (b) an intermediate eutrophic area (Z. noltii absent, although residual roots can still be found in the sediment, and the occasional formation of abundant macroalgae mats) and (c) a strongly eutrophic area (macrophyte community totally absent for at least a decade and strong, regularly occurring, blooms of Ulva spp.…”

“…Whenever possible, production/biomass ratios (P/B), previously calculated for local populations (e.g. Marques et al, 1994;Lillebø et al, 1999;Pardal et al, 2000;Ferreira et al, 2004), were used. When this was not feasible, values taken from the literature (Appendix A) were utilised.…”

Enhanced erodibility of fine-grained marine sediments by Hydrobia ulvae

“…The common mud snail Hydrobia ulvae is a widespread deposit feeder at fine-grained sediments on European intertidal flats and an evaluation of the effect of this species on the erodibility of the bed is therefore of major importance. The snail may occur in densities of up to about 300 000 individuals m À 2 (Blanchard et al, 1997;Austen et al, 1999;Jensen, 1992;Lillebø et al, 1999). Some studies (Blanchard et al, 1997;Andersen, 2001a) have indicated that H. ulvae increase the erodibility of their substrata.…”

The faunal role in the degradation of the common intertidal salt marsh plant Scirpus maritimus

“…Microcosmos experiments have suggested that, even at low densities, nematodes can have a significant structuring top-down influence on the bacteria growing on the cordgrass leaves detritus (DeMesel et al, 2004). Macrofauna may also feed directly on bacteria, and species such as Hydrobia ulvae, through their feeding activities on leaf surfaces (Barnes & Hughes, 1988;Lillebø et al, 1999b), might contribute to the stimulation of microbial activity. Moreover, it has been shown that, in the presence of meiofauna, polychaetes can incorporate significantly more carbon from Zostera detritus (Tenore et al, 1977in Valiela, 1995.…”