Effects of algal turf canopy height and microscale substratum topography on profiles of flow speed in a coral forereef environment

Abstract

Water motion can be an important variable affecting the recruitment, metabolism, growth rates, feeding efficiency, and distribution of aquatic organisms. Recent studies suggest that flow speed may be an important factor regulating primary productivity of reef algal turf communities. We measured profiles of flow speed over algal turfs with different canopy heights and over algal turfs growing on substrata that varied in microtopography. Profiles over sea urchin‐grazed algal turfs with low canopy heights and algal turfs growing on topographical high points indicate that the boundary layer is dominated by turbulent eddies. In contrast, flow over algal turfs with higher canopy heights and those growing within depressions in the substratum is influenced more by viscous forces that may lead to diffusion‐limited metabolism of these algal communities. These data demonstrate that flow speeds over coral reef algal turfs exhibit spatial variation over small spatial scales and temporal variation that is dependent on ambient flow speeds. Our results predict microscale variation in algal turf metabolism and suggest that photosynthetic rates of this component are likely to be below maximum capacity for a significant portion of the time.

Evidence for water-mediated mechanisms in coral–algal interactions

“…Furthermore, our study shows that thick turf oriented upstream retains the hyper-and hypoxic conditions at the coral-algal interface more strongly compared to thin turf, supporting the importance of algal canopy height, flow speed, and orientation in watermediated interactions between corals and algae. Elevated turf algae are known to create thick DBLs [23]. They might create a thick DBL at the downstream interface with corals by acting as a physical barrier, especially in low flow conditions.…”

“…These studies suggest that hydrodynamics limit the impact of algal exudates in the natural environment and that water-mediated interactions between corals and algae largely operate at the mm-to-mm scale within the DBL. The benthic DBL is a thin film of stagnant, diffusionlimited water surrounding the benthos, whose thickness is determined by flow and microtopography [23,24]. Thinner DBLs promote the exchange of dissolved gases and nutrients for metabolic processes and the removal of wastes [25,26].…”

Effects of nutrients versus herbivores on reef algae: A new method for manipulating nutrients on coral reefs

“…More importantly for the models being assessed, the relative dominance model predicts that, other things being equal, turf dominance should be enhanced by lower rather than higher nutrient regimes. Because filamentous turf-forming algae have high growth rates, have a significant portion of their total biomass in the benthic boundary layer, and may have nutrient uptake limited due to boundary layers produced by closely spaced upright filaments (Carpenter and Williams 1993), growth of these algae may be limited by nutrients more than is the growth of larger macroalgae. If this hypothesis is correct, then portions of the relative dominance model would need revision.…”

“…The growth rates of reef seaweeds are believed to be constrained by nutrient limitation, thereby preventing seaweeds from overgrowing and killing corals under oligotrophic conditions typical of tropical reefs (Birkeland 1988;Littler et al 1991;Lapointe 1997). Another important factor in this balance of coral/seaweed competition is the high rate at which seaweeds are consumed by abundant and diverse coral reef herbivores (Carpenter 1986;Lewis 1986;Hay 1991). More than a decade ago, Littler and Littler (1984) recognized the interdependence, complexity and context-specific nature of factors determining coral reef community structure and proposed the relative dominance model to predict changes in reef communities as a function of interactions between herbivory and nutrient levels.…”

Gypsum dissolution is not a universal integrator of ‘water motion’

“…Gundersen et al (1992) found that fluctuations in flow, though not as regular, significantly decreased diffusive boundarylayer thickness and could increase the nutrient and oxygen supply to a hydrothermal vent bacteria mat. Carpenter and Williams (1993) found that oscillatory flow enhanced reef algal turf photosynthesis by 20% over unidirectional flow and suggested that turbulence resulting from the reversal of flow direction disrupted the diffusive boundary layers.…”