Abundances of macrobenthic species were monitored twice yearly (March and September) at 6 locations in Tees Bay, UK, between 1973 and, and once yearly at 4 stations in the outer Tees estuary and 7 stations in the inner estuary between 1980 and 1999. In the Bay, multivariate analysis revealed a serial pattern of community change over years for all areas, but with a major shift in community composition between 1986 and 1988. Inter-annual variability in community composition was significantly greater after 1987 than before 1987 in all areas. Overall, inter-annual variability was greater in areas near the estuary mouth than in areas farther away, although the direction of community change and the timing of the discontinuity were the same in all areas. The serial nature of community change with time was also weaker in the areas close to the estuary mouth. Although there was no clear pattern of change in the number of species present over the sampling period, a dramatic increase in Shannon diversity (H ') occurred after 1987, due to an increase in evenness that resulted from the reduction of a few previously dominant species, notably the small polychaete Spiophanes bombyx. Although biodiversity measures describing the taxonomic breadth of the species assemblages also showed a marked step change in 1987, this was one of reduced diversity, with average taxonomic distinctness (∆ + ) decreasing and the variation in taxonomic distinctness (Λ + ) increasing. These abrupt, detrimental changes coincided with a well-documented change in a variety of components of the North Sea ecosystem during the same period. Traditional species diversity measures, such as H ', therefore gave a false impression of improving environmental quality over this period: given that the average taxonomic spread was reduced, certain taxa were under-represented with respect to others, and community composition as measured by a multivariate stability index (MSI) became less stable. H ' also failed to distinguish putatively impacted areas close to the estuary mouth compared with those more distant, despite clear differences in ∆ + , Λ + , and in community stability (MSI). Overall patterns of biodiversity and community composition in the Bay have thus been affected temporally by regional changes in the North Sea ecosystem, and spatially by the effects of the estuarine outflow. In the estuary itself, multivariate analysis also revealed a serial pattern of community change, with a major shift in composition in 1994 in both the outer and inner estuary which coincided with the construction of a barrage in the estuary. The numbers of both individuals and species began to increase at this time in the outer estuary. H ' showed no obvious changes over the period, but in the outer estuary a step change in ∆ + and Λ + occurred at the same time as that in the Bay. However, the direction of change was the reverse of that in the Bay, suggesting an improvement in environmental quality or a shift to more saline conditions.

The relative influences of fine and landscape scale factors on the structure of lentic plant assemblages. Lake Reserv Manage. 32:116-131. The process of plant community assembly has long been a topic of debate among ecologists. Aquatic plant assemblages and their structure may be the result of a series of abiotic and biotic filters that include transport (i.e., physical movement of species), water chemistry, sediment chemistry, basin structure, and competitive interactions. The influences of transport and water chemistry have been well investigated, but many questions persist about the interrelationships among water and sediment variables, including their combined influences on the structure of the aquatic plant assemblage. To understand how these abiotic conditions interact, we sampled 750 points in 30 lakes. Using these data, a split canonical correspondence design was used to evaluate the relative influences of regional and fine-scale conditions in structuring the plant assemblage. Additionally, multiple logistic regressions were employed to determine the individual species' abiotic preferences. The results suggest that the plant assemblage structure is principally determined by the lake-sediment characteristics. Sediment variables explained 28% of the total species-data variance and water chemistry accounted for 9% (total variance explained = 46%; 9% due to intercorrelation of water/sediment characteristics). There were also strong species-environmental relationships; nonnative species showed distinct correlations with sediment pH and lake water conductivity, which may be useful in lake management initiatives such as risk assessment and nonnative species monitoring. Finally, and as expected, the Shannon diversity index was strongly related to depth and light. Assembly theory views plant communities as aggregates of species present in an area due to their relationships with environmental conditions, successful recruitment, and competitive ability (Weiher and Keddy 1995, Wilson 1999). Plant assemblage structure and the inherent oscillations of species vary with abiotic conditions and disturbances but may result in an assemblage composed of species best suited to the local environment (Weiher and Keddy 1995, Wilson 1999). In this study, we examined the composition of rooted aquatic plant assemblages and the associated abiotic filters to better understand the mechanisms that structure these assemblages in lentic systems. In 2009, Capers et al. (2009a) examined the influences of lake location, water chemistry conditions, and the dispersal abilities of species on the structure of these