Species point records from 1975-78 University of Bristol Severn Estuary littoral rock survey

Zonation of algae and invertebrates was examined on rocky shores in the Severn estuary from 1975 to 1978. These shores were characterised by an absence of macro-algae from MLWS and below. Some littoral algae showed reductions in abundance during the study. In particular, Ascophyllum nodosum and Fucus serratus almost disappeared from the mid-shore at one site after 1976, and no significant recolonisation had occured by 1979. Animal species in general showed predictable and constant zonation patterns, but Littorina rudis, Patella vulgata and Eliminus modestus all exhibited decreased upper vertical limits in the upper reaches of the estuary. Three approaches were taken to interpret the zonation patterns of their changes. 1. Exposure to wave action was estimated. Decreasing exposure levels may account for the decline in upper vertical limits of species in the upper estuary. 2. A survey of silt, salinity and temperature was carried out over compelte tidal cycles. High silt concentrations may partially explain the absence of macro-algae below MHWS. 3. Preliminary experiments compared the effects of grazers in the estuary with those in fully marine conditions. These experiments point to some of the factors which are responsible for the lack of algal recolonisation on the shore from which the dominant fucoids disappeared after 1976, but the situation is complex and involves algal reproductive biology as well as grazer activity.

自1975年至1978年，对塞文河口岩石海岸的藻类和贝类动物的群落分区进行了考察。这些海岸的特征在于从最大低潮线（MLWS）以下区域缺乏大型藻类。在研究期间，一些沿岸藻类的丰度有所下降。特别是，在1976年之后，Ascophyllum nodosum和Fucus serratus在某一中岸位置几乎消失，并且到1979年，并未观察到显著的再定居。动物物种总体上显示出可预测且恒定的分区模式，但Littorina rudis、Patella vulgata和Eliminus modestus在河口上游均表现出垂直上限的下降。采用了三种方法来解释这些变化模式的分区特征。1. 对波浪作用的暴露程度进行了估算。减少的暴露水平可能解释了上游河口物种垂直上限的下降。2. 在完整潮周期内对泥沙、盐度和温度进行了调查。高泥沙浓度可能部分解释了在高潮线以下区域大型藻类的缺失。3. 初步实验比较了河口中食草动物与完全海洋条件下的影响。这些实验指向了一些负责1976年后消失的占主导地位的硅藻类海岸上藻类未再定居的因素，但情况复杂，涉及藻类的繁殖生物学以及食草动物的活动。