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The aim of this field study was to investigate effects of estimated fish-and sea urchin herbivory on the reproductive potential of four species of macroalgae; Halimeda macroloba (Decasine), H. renschii (Hauck), Turbinaria ornata (Turner) and Padina boergesenii (Allender et Kraft). Fish and sea urchin herbivory were calculated based on reported consumption rates for their biomass estimates. We hypothesized that reduced herbivory would increase algal size and the reproductive potential, which may promote algal recruitment and be one of the driving mechanisms behind algal shifts and persistent algae-dominated reefs. Algae were investigated in field sites where the estimated fish-and or sea urchin herbivory differed. Our results suggest that algal fecundity of T. ornata and P. boergesenii are positively correlated to their size. Fecundity of T. ornata was higher and individuals grew larger in areas where estimated fish herbivory was lower. The two species of Halimeda grew larger and had higher fecundity in areas where estimated sea urchin herbivory was lower. P. boergesenii responded ambiguously to patterns in herbivory. Due to species-specific responses to different herbivores, it is difficult to generalize about effects of overfishing on algal fecundity.

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Abstract:Eutrophication is an increasingly occurring problem that causes degradation of coastal ecosystems worldwide. Traditional monitoring of dissolved nutrients in the water column can be complicated by short term fluctuations, especially when levels of nutrients are low and turnover rapid. A proposed alternative method is the use of macroalgal tissue nutrient concentrations for indication of ambient nutrient availability, as they integrate nutrients over time. This study evaluates the use of macroalgae within the genus Ulva (Chlorophyceae), regarding their ability to reflect nutrient gradients off the coast of East Africa (i.e. Kenya and Tanzania). Ulva was able to reflect nitrogen (N) gradients from all three outlet sources, with up to 90% higher tissue N levels in vicinity of major cities compared to adjacent areas. Together with results from nutrient-uptake and fluctuation experiments, this study suggests that macroalgal tissue nutrient content has potential as a complement to regular water nutrient sampling.