Seasonal Movements of Subtidal Benthic Communities in the Fraser River Estuary, British Columbia, Canada

Abstract

The subtidal benthic fauna of the lower Fraser River, a salt-wedge estuary, was sampled monthly from June 1977 to August 1978 in mud substrates at six stations ranging from oligohaline to polyhaline. Subtidal interstitial salinities were also measured and were related to the seasonal distribution of the estuarine benthic fauna. Diurnal comparisons of sediment salinities with those of the immediately overlying water showed that interstitial salinites of sediments containing a high proportion of silt vary seasonally, not diurnally. Thus, there is a transition zone between salt and fresh interstitial water that is clinally shifted up and downstreams with seasonal variations in freshwater discharge. Because of this, seasonal shifts occur in the distribution and demography of benthic infaunal species. Seasonal shifts in relation to interstitial salinity variations are shown to occur in the distribution of the oligochaetes Limnodrilus hoffmeisteri, Tubifex tubifex, Tubificoides gabriellae and Paranais litoralis and of the polychaetes Eteone longa, Amphicteis sp. and Polydora kempi japonica. These seven species comprised over 25% of the total taxa collected and over 60% of the individuals collected. The data on other species distributions support the premise of cyclic changes related to seasonal interstitial salinities but this is not always shown clearly because sampling was biased towards the most common oligochaete populations. The system is stable and it is suggested that seasonal faunal shifts are a feature of salt-wedge estuaries in general. In addition, vertical salinity gradients exist in sediments and these change seasonally.