Modelling the effects of salinity variation on Echinogammarus marinus Leach (Amphipoda, Gammaridae) density and biomass in the Mondego Estuary (Western Portugal)

Echinogammarus marinus (Amphipoda. Gammaridae) is the most abundant amphipod species in the intertidal rocky habitats of the Mondego estuary and it seems to constitute an important prey for upper trophic levels. The aim of the present model was to assess the impacts of salinity variations on the abundance and biomass of E. marinus. For model purposes. the size structure of the population was divided in four different groups: juveniles. young females, adult females and males, which constitute the model state variables expressed as density (number of individuals m(-2)). The variation of the density of each group over time depends on growth, mortality and reproduction recruitment. Simultaneously, the model also calculates the biomass of each group, by taking into consideration the density and the average weight of each group through time. The model accounts for a dependency of population density on biomass, since the number of newly hatched juveniles depends on female cephalic length and, consequently, on female size. Population growth is estimated according to a Von Bertalanffy model, which takes into account anabolism and catabolism effects. Moreover, E. marinus growth is considered as temperature-dependent, while recruitment is considered as temperature- and salinity-dependent. Model results suggest that E. marinus density is seriously affected by low salinity ( < 5) and that the negatives effects increase with increasing time of exposition to low salinity. According to the model, if low salinity persists during extensive periods (autumn/winter/spring) E. marinus population will extinguishes after 3 years, under such conditions. According to simulations, if low salinity occurs for shorter periods (winter/early spring) E. marinus biomass and density remains stable for, at least, 11 years. This type of models may be important in assessing the impacts on local coastal areas caused by global effects, such as the rising of global temperature, which is expected to rise sea level and change precipitation and other local climate conditions. (C) 2002 Elsevier Science B.V. All rights reserved.