Shallow water assemblages exposed to abiotic disturbance & stress : the structuring role of emersion and UV-radiation
Using field experiments, I tested the applicability of the “Intermediate Disturbance Hypothesis” (IDH). Diversity H` showed a unimodal response for both disturbance qualities, evenness and species richness only along the emersion and enhanced UVB gradient, respectively. However, response patterns along disturbance gradients were inconsistent and the unimodal pattern non-persistent. By extending Connell´s IDH model, I conclude that a reduction in diversity of emersion treatments under harsh conditions results from a shift in dominance, to species well adapted for that disturbance quality, rather than loss of species. Enhanced UVB radiation generated a unimodal response for diversity at a time when emersion did not, suggesting different mechanisms between both disturbance qualities in producing the same pattern. The role of the green alga Enteromorpha intestinalis was discussed in that respect. Effects of ambient UV stress were non-persistent, both at the SE Atlantic and Baltic Sea study site. UVA radiation alone was without effect on fouling assemblage structure at both sites. Already ambient UV radiation lowered diversity H` and species richness, but not evenness, at the SE Atlantic site, while enhanced UVB reduced diversity H`, species richness and evenness at the Baltic Sea site. The role of protective shading under the canopy of UV-resistant algae (Enteromorpha intestinalis at the Baltic Sea site and Ceramium sp. at the SE Atlantic site) for the generation of non-persistent effects was discussed in that context. Inter-active effects between UV radiation and consumers were almost absent at both study sites and in addition, non-persistent for the rare occasions when they occurred. In conclusion, my results suggest the applicability of the IDH, but the unimodal response for diversity was (i) modified by inter-annual variability in recruitment and growth of dominant species, (ii) non-persistent in succession, (iii) dependent on the chosen disturbance quality and (iv) diversity parameter. A non-persistent ecological relevance of UV stress was shown for two distinct fouling assemblages, grown under different natural irradiance regimes.