The effect of sewage discharge on <em>Melarhaphe neritoides</em> (Gastropoda: Littorinidae) population dynamics

Abstract

SuMMarY: The discharges of sewage effluent treatment plants has a major impact on coastal communities. in our study area (western coast of Portugal) Melarhaphe neritoides (l. 1758) is the dominant high-shore gastropod. Two populations of M. neritoides were studied in order to understand the impact of sewage discharges on intertidal communities: one population in an impacted area and the other in a similar but unimpacted area (reference site). environmental data and abundance, biomass, population structure and annual growth production of M. neritoides were estimated in the two areas. The results showed that M. neritoides density is higher in the impacted area as a result of massive settlement. The sewage discharge increases the nutrient concentrations in the water, which causes more microalgae to grow on the rocky surfaces. This higher availability of food may promote recruitment. however, we found high mortality in the juveniles in the impacted area, which affected density values on the upper shore levels (where we found the adults) of both areas. Moreover, the adults were bigger in the unimpacted area, which suggests that individuals grow more or live longer in this area.Keywords: Melarhaphe neritoides, abundance, biomass, secondary production, sewage, nutrient enrichment, rocky shore. reSuMen: Efecto de la descarga de aguas residuales sobre la dinámica poblacional de Melarhaphe neritoides (Gastropoda: Littorinidae). -las descargas de aguas residuales de depuradoras son uno de los mayores impactos sobre las comunidades costeras. en nuestra área de estudio (costa oeste de Portugal) Melarhaphe neritoides (l. 1758) es el gasterópodo dominante en la zona supralitoral. Para entender el impacto que estos vertidos tienen sobre las comunidades intermareales, estudiamos dos poblaciones de M. neritoides: una en un área contaminada y otra en una zona similar, pero no impactada (zona de referencia). en ambas áreas se registraron datos medioambientales, datos de abundancia y biomasa de M. neritoides y se estimó la estructura de las poblaciones y la producción anual. los resultados muestran una mayor densidad de M. neritoides en la zona contaminada, como resultado de una colonización masiva. las descargas de aguas residuales aumentan la concentración de nutrientes en el agua, provocando un mayor crecimiento de microalgas en la superficie de las rocas, lo que facilita la colonización. Sin embargo, verificamos una elevada mortalidad de juveniles en la zona contaminada que aproximó los valores de la densidad de las poblaciones en las zonas más altas del litoral (donde encontramos los adultos) en ambas zonas. además, los adultos eran de mayor tamaño en las áreas no contaminadas, lo que sugiere que en esta zona los individuos tienen un mayor crecimiento o viven durante más tiempo.

Effects of sewage pollution on the structure of rocky shore macroinvertebrate assemblages

“…However, to assess the effect of sewage discharges on the population of M. neritoides it is necessary to study the population along all tidal height, and not only in the littoral fringe. Planktonic larvae settle on the lower levels of rocky shores and then start moving upshore, resulting in a shell size gradient (Cabral-Oliveira et al, 2009). The results obtained in this work confirmed previous findings (Cabral-Oliveira et al, 2009) where the density of M. neritoides was higher in the impacted area as a result of massive settlement.…”

“…Planktonic larvae settle on the lower levels of rocky shores and then start moving upshore, resulting in a shell size gradient (Cabral-Oliveira et al, 2009). The results obtained in this work confirmed previous findings (Cabral-Oliveira et al, 2009) where the density of M. neritoides was higher in the impacted area as a result of massive settlement. Also in this work was found a higher number of juveniles in the eulittoral near the outfall.…”

Are eulittoral assemblages suitable for detecting the effects of sewage discharges in Atlantic and Mediterranean coastal areas?

“…On both shores (Atlantic and Mediterranean), impacted locations were characterized by the presence of an outfall discharging effluents directly on the rocky shore in the intertidal zone. Previous studies have already reported the impact of sewage discharges on macrofauna, meiofauna and fish assemblages in these areas (Guidetti et al 2003;Terlizzi et al 2005b;Fraschetti et al 2006;Cabral-Oliveira et al 2009). Both outfalls release secondary treated effluents from a human population of 30,000 to 40,000.…”