Worldwide, coral reef communities are collapsing and shifting from coral to algal dominance. While human activities changing top-down (overfishing of grazers) and bottom-up (increases in nutrients and sediments) forces can initiate shifts, complex, non-linear interactions among stressors limit predictability (Hughes et al., 2007;Bellwood et al., 2004). Further, while top-down control is known to limit algal dominance (Knowlton & Jackson 2008; Jackson et al., 2001), less is known about bottom-up controls that may facilitate algae. One functional group of algae is turf algae, which are multi-species and often filamentous, ranging from 0.01-10cm in height (Fong & Paul 2011). Short, closely-cropped turf algae provide ecosystem functions such as primary productivity, trophic support, and nutrient cycling (Fong & Paul 2011). Under environmental stress, however, turf algae can shift to long sediment-laden turf, which may be an alternative stable state that inhibits coral recovery (Adjeroud et al., 2009).
My first objective was to compare variation in the assembly of turf algae under a suite of top-down and bottom-up contexts at six sites on fringing reefs in Mo’orea French Polynesia. At each site, 10 settlement tiles were deployed and monitored for two-months and then after twelve months. During the first two months I also measured herbivore abundance, grazing pressure, sediment composition, nutrient availability, benthic community structure, and sediment deposition rates at each site. After two months turf did not vary among plots, despite significantly different herbivore abundances and nutrient availability. This implies that early successional forms are easily controlled, even by low numbers of herbivores, due to their high palatability. In contrast, after one year, tiles in highly grazed sites with low sedimentation rates were dominated by crustose coralline algae, and sites with low herbivores and high sedimentation rates dominated by long turf and macroalgae. As crustose coralline algae can facilitate coral recruitment, these results imply that herbivory can mediate the successional trajectory of algal communities toward recovery by coral or stability of the shifted algal state.
A robust herbivorous fish community is the paradigm of a healthy coral reef, however, altered sediment regimes can negatively influence their grazing patterns (Tebbett et al., 2018;Bellwood & Fulton 2008), leading to algal proliferation. While increases in sediment can lead to algal proliferation through reduced grazing and release from nutrient limitation, too much sediment can lead to negative turf responses (Tebbett et al., 2018) due to the buildup of anoxic-inducing bacteria (Clausing et al., 2014). Turf are particularly prone to the influence, both positive and negative, of sediment fluxes due to their filamentous nature that trap deposited sediment (Rogers 1990). The second objective of my dissertation was to establish the non-linear relationship between sediment and turf at two sites that varied in environmental context. Before manipulations commenced I conducted field surveys of sediment depth and turf height at seven fringing reef sites around Mo’orea. I found that sediment depth and turf height varied widely among sites and although longer turf did hold more sediment that shorter turf communities, it was not disproportionally more than expected. I then choose two sites that varied the greatest in turf and sediment and deployed tiles where I manipulated sediment depth (0,1,3, & 5mm) for 25 days. Because all tiles were open to herbivory, I quantified herbivorous fish communities through visual surveys. Interestingly, I found that sediment additions have overall negative impacts on turf, and that turf between sites did have different thresholds for sediment. As turf becomes a more conspicuous component of reefs worldwide, susceptible to increases in terrestrial fluxes, understanding the dynamics of this relationship will become crucial for predicting reef recovery and resilience.
In the Pacific, a healthy coral reef is characterized by intact herbivorous fish communities that reduce shifts to algal dominance through grazing (Hughes et al., 2007;Mumby et al., 2006). Sea urchins are an alternate grazing guild that are less studied, especially in the Pacific, but were found to maintain healthy algal communities on Caribbean reefs (Mumby et al., 2007;Lewis et al., 1987;Carpenter 1986; Carpenter 1985). My third objective was a novel exploration of the role urchins play in limiting algal proliferation in the South Pacific, and how this may be mediated by sediment, as sediment is known to deter fish grazing. Sediment (no addition, 3mm marine, or 3 mm terrestrial/marine mix) and presence of urchins (+/-) were manipulated in plots containing turf algae on a relatively overfished reef when compared to other Mo’orean fringing reefs (data from Mo’orea Long Term Ecological Research Project). I found urchins maintained closely cropped turf even under high sediment conditions. Further, urchins removed added sediment, likely promoting consumption by fishes. Thus, urchins can be functionally redundant grazers as they can compensate for the loss of herbivorous fishes. Overall, turf is a dynamic community, sensitive to small environmental shifts dictating abundance, proliferation, and taxonomy, potentially altering its function in coral reefs.
