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Species richness patterns are remarkably similar across many marine taxa, yet explanations of how such patterns are generated and maintained are conflicting. I use published occurrence data to identify previously masked latitudinal and longitudinal diversity gradients for all genera of benthic marine macroalgae and for species in the Order Bryopsidales. I also quantify the size, location, and overlap of macroalgal geographic ranges to determine how the observed richness patterns are generated. Algal genera exhibit an inverse latitudinal gradient, with biodiversity hotspots in temperate regions, while bryopsidalean species reach peak diversity in the tropics. The geographic distribution of range locations results in distinct clusters of range mid-points. In particular, widespread taxa are centered within tight latitudinal and longitudinal bands in the middle of the Indo-Pacific and Atlantic Oceans while small-ranged taxa are clustered in peripheral locations, suggesting that variation in speciation and extinction are important drivers of algal diversity patterns. Hypotheses about factors that regulate diversity contain underlying assumptions about the size and location of geographic ranges, in addition to predictions as to why species numbers will differ among regions. Yet these assumptions are rarely considered in assessing the validity of the prevailing hypotheses. I assess a suite of hypotheses, suggested to explain patterns of marine diversity, by comparing algal-richness patterns in combination with the size and location of algal geographic ranges, to the richness and range locations predicted by these hypotheses. In particular, the results implicate habitat areas and ocean currents as the most plausible drivers of observed diversity patterns.

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The response of the scleractinian coral Stylophora pistillata to short-term (min to h) and long-term (d) exposure to reduced-salinity seawater was examined. Pulse Amplitude Modulated (PAM) chlorophyll fluorescence techniques were used to assess the photosynthetic efficiency of the symbiotic dinoflagellates (dark-adapted F v /F m ) in the coral tissues (in hospite) before, during and after exposure. Exposure to reduced-salinity seawater caused a marked reduction in efficiency (the ratio of variable [F v ] to maximal [F m ] fluorescence), and there was an apparent link between a reduction in dark-adapted F v /F m and a loss of symbiotic dinoflagellates from the corals. The reduction in F v /F m of the symbiotic algae and subsequent dissociation of the coral-algal symbiosis (coral bleaching) occurred during exposure to reduced-salinity seawater in either the light or dark. The results demonstrate that bleaching in response to low-salinity seawater is a truly sublethal response, contrary to a recent suggestion. The study also suggests that bleaching of corals in response to low-salinity seawater may not involve the passive loss of algal symbionts and that an impairment of the capacity of the algal symbionts for photosynthesis represents a common 'cue' initiating the dissociation of the coral-algal symbiosis during exposure to sub-optimal conditions. This study demonstrates how exposure to low-salinity seawater alone can cause some of the symptoms commonly attributed to temperature anomalies and anthropogenic pressures on coral reefs.