A testable linear model for diversity trends in estuaries

Summary In 1934, Adolf Remane constructed a diagram to describe changes in the number of species along a full salinity gradient within the Baltic. Despite fundamental differences in tidal regimes, the Baltic model has been applied directly to estuaries, becoming subsequently the textbook model for estuarine diversity trends. Despite its ubiquity, the Remane diagram has many inconsistent features, making it unsuitable as a quantitative tool for comparing diversity trends between estuaries, including poor definition of x ‐axis (salinity), y ‐axis (number of species) and variations in sample location (subtidal/intertidal) than can greatly influence the resulting diversity/salinity relationship. Consequently, diversity trends within and between estuaries remain to be tested robustly. The major environmental factor influencing the distribution of organisms in estuaries is salinity variation, rather than absolute salinity tolerance as in the Baltic. As salinity range demonstrates a quadratic relationship with mean salinity, an alternative linear model is therefore suggested utilizing mean salinity range at any one point in the estuary ( x ‐axis) and mean α‐diversity of macroinvertebrates ( y‐ axis) obtained from subtidal samples to allow comparison with river systems and to minimize salinity variability between interstitial and overlying water. The model was tested on an extensive subtidal data set from the Thames estuary (salinity 0–35), significant negative linear relationships between salinity range and α‐diversity being apparent for annual and seasonal data sets. Significant models were also possible for both ‘marine’ and ‘freshwater’ halves of the estuary and for meiofauna α‐diversity. The linear model allows formal, statistical investigation of the differences in diversity trends between estuaries and the development of testable hypotheses on aspects of estuarine diversity, including the causes of the species minimum in estuaries. It also has potential as a management tool enabling the definition of sites of concern that fall below their predicted diversity. The new model of diversity now requires testing in systems additional to the Thames in order to determine whether this relationship is a macroecological phenomenon or restricted to the test system.