Capturing the dynamics in benthic structures: environmental effects on morphology in the macroalgal genera Halimeda and Dictyota

Macroalgae are modular organisms that can express different morphologies depending on the environment to which they are exposed. Their growth under varying light, temperature, and nutrient regimes, interacting with disturbance factors such as herbivory and hydrodynamics, leads to particular morphological types. The present study illustrates the potential of using morphological variations of benthic modular and/or clonal organisms as indicators of the factors and processes influencing them in their particular location. The morphogenetic agent-based model SPREAD (spatially explicit reef algae dynamics) was used to determine the range of potential morphological types in 3 dominant macroalgal species (Halimeda tuna, H. opuntia, and a species of Dictyota) in the Florida Reef Tract. Simulations of growth under a range of light, nutrient, and disturbance conditions similarly found at inshore patch and offshore bank reefs led to 6 potential morphological types for H. tuna, 2 for H. opuntia, and 3 for a species of Dictyota. From these potential sets derived from the model, we observed that particular morphological types corresponded to the morphologies found in the 2 reef habitat types. The simulated conditions that led to the formation of these morphologies in the model were similar to the environmental conditions at these sites. In addition to relating combinations of environmental and disturbance factors to macroalgal growth morphologies, the present study provides insights into the differing life-history strategies among the species, and the adaptive value of plasticity expressed by these macroalgae. The morphologies of the successful fragmenters H. opuntia and a generalized species of Dictyota were more influenced by disturbance. On the other hand, the morphology of the less successful fragmenter H. tuna was strongly influenced by the growth factors of light and nutrients.