Algal blooms on coral reefs: What are the causes? (original) (raw)
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Limnology and Oceanography, 1997
During the past two decades coral reefs in the greater Caribbean area have been altered by phase shifts away from corals and toward macroalgae or algal turfs. This study tested the hypothesis that because the phase shift on reefs in Jamaica and southeast Florida involved frondose macroalgae, bottom-up control via nutrient enrichment must be a causal factor. The approach was multifaceted and included measurement of near-bottom nutrient concentrations, salinity, nutrient enrichment bioassays, alkaline phosphatase assays, tissue C : N : P ratios, and tissue 15N : 14N (6"N) ratios. In both locations, concentrations of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) exceeded nutrient thresholds (-1 .O PM DIN, 0.1 PM SRP) noted to sustain macroalgal blooms on Caribbean coral reefs. High seawater DIN : SRP ratios, alkaline phosphatase activity, and tissue C : P and N : P ratios of macroalgae on the carbonate-rich Jamaican reef suggested SRP limitation of productivity compared to lower values of these variables on siliciclastic reefs in Florida that suggested DIN limitation. This pattern was corroborated experimentally when SRP enrichment increased P,,,, (photosynthetic capacity at light saturation) of the chlorophyte Chaetomorpha Zinum in Jamaica compared to DIN enrichment that increased (x (the photosynthetic efficiency under low irradiance) of the deeper growing chlorophyte Codium isthmocladum in southeast Florida. Increased DIN concentrations were associated with reduced salinity on both reefs, indicating submarine groundwatcr discharge was a significant source of DIN. Elevated S15N values of C. isthmocladum tissue further pointed to wastewater DIN as a source of nitrogen contributing to the blooms in southeast Florida.
Limnology and Oceanography, 1999
Is herbivore loss more damaging to reefs than hurricanes? Case studies from two Caribbean reef systems (1978-1988), p. 220-226. In R. Ginsburg [ed.], Proceedings of a colloquium on global aspects of coral reefs: Health, hazards and history. Univ. of Miami. SZMANT, A. M. 1997. Nutrient effects on coral reefs: A hypothesis on the importance of topographic and trophic complexity on nutrient dynamics. Proc. 8th Int. Coral Reef Symp. 2: 1527-1532. , AND A. FORRESTER. 1996. Water column and sediment nitrogen and phosphorus distribution patterns in the Florida Keys, and potential relationships to past and present coral reef development. Coral Reefs 15: 21-41. VAN DEN HOEK, C., A. M. BREEMAN, R. P. M. BAK, AND G. VAN BUURT. 1978. The distribution of algae, corals and gorgonians in relation to depth, light attenuation, water movement and grazing pressure in the fringing coral reef of Curacao, Netherlands Antilles. Aquat. Bot. 5: 1-46. WOODLEY, J. D. 1999. Diadema exerts top-down control of macroalgae on Jamaican coral reefs. Coral Reefs 18: In press.
Eutrophication and Changes in Algal Growth on Coral Reefs
Freshwater runoff naturally delivers large amounts of nutrients to the near shore environment resulting in macroalgae proliferation. Changes in land use (e.g. agriculture, urbanization, etc.) have dramatically increased nutrient levels with a corresponding change in the abundance of macroalgae. Using the Viapahu River (Moorea, French Polynesia) as our study system, we tested the hypothesis that riverine input affects macroalgal abundance and composition. Nutrient levels in our sample site changed significantly with riverine enhancement. There was a significant difference in the levels of total nutrients before and after the rainy season began, although the overall abundance and spatial location of algae did not change significantly. The site composition significantly changed with riverine input and there was a significant difference in the overall substrate composition of the site after the rains began. Algal growth plates were set out to monitor growth. Picture analysis of the plat...
Macroalgal blooms on southeast Florida coral reefs
Harmful Algae, 2005
Invasive blooms of the siphonaceous green algae Codium spp. have been considered a symptom of coastal eutrophication but, to date, only limited biochemical evidence supports a linkage to land-based nutrient pollution. Beginning in the summer of 1990, spectacular blooms of unattached Codium isthmocladum developed on deep coral reef habitats in southern Palm Beach County and northern Broward County, and in subsequent years, attached populations formed on reefs in northern Palm Beach County.
Bulletin of Marine Science, 2013
Populations of Diadema antillarum (Philippi, 1845) have been slow to recover after their 1983 Caribbean-wide massive mortality. The continued absence of this important coral reef herbivore as well as further impacts on local and regional-scales have severely threatened the health of coral reefs. From 2008 to 2011, the population of D. antillarum and structure of the benthic community were monitored within Akumal Bay, Mexico, a heavily impacted ecosystem. Across the 4 yrs of the study, densities of adult D. antillarum did not significantly change and were highest on backreef sites (mean 1.30 and 1.44 m −2). Coral cover was low at all sites (maximum 14%) and decreased during the 4 yrs of the study. Changes in crustose coralline algae (CCA) cover varied by year but was significantly higher on forereef sites, while cover by turf-algal-sediment (TAS) increased at all four sites. Diadema antillarum densities correlated positively with coral and CCA cover in the backreef sites where sea urchin densities were higher but no correlation occurred with TAS and turf. Where Echinometra lucunter (Linnaeus, 1758) sea urchins were more prevalent, their densities correlated negatively with coral and positively with CCA. Although densities of adult D. antillarum were comparable to other regions of the Caribbean, increases in TAS over the 4 yrs of this study may be evidence that even an important herbivore cannot prevent continued decline of a reef in an impacted ecosystem. Coral reefs in the Caribbean have experienced major disturbances over the past few decades and continue to be under a combination of stressors including bleaching, overexploitation of herbivores, coral diseases, and declining water quality, which has resulted in an increase in macroalgae cover (Knowlton 1992, Aronson and Precht 2001, Hughes et al. 2003, Bellwood et al. 2004). The importance of long-spined sea urchin, Diadema antillarum (Philippi, 1845), in removing macroalgae cover became evident after their massive mortality in 1983, when algal biomass abruptly increased (de Ruyter van Steveninck and Bak 1986, Carpenter 1988, Hughes 1994). As the populations of D. antillarum recover after their 1983 Caribbean-wide massive mortality (Lessios et al. 1984b, Lessios 2005), it remains unclear if grazing by this herbivore provides a suitable driver from which to expect a decrease in macroalgal cover on impacted coral reefs. Modeled scenarios indicate that sea urchins impart significant resilience to Caribbean reefs such that continued decline is inevitable when sea urchins are scarce (Mumby et al. 2006). However, abundant literature indicates it is a combination of factors which impacts the benthic dynamics on coral reefs and the propensity for shifts between dominant states [e.g., competition for space (Sandin and McNamara 2012), herbivore diversity (Cheal et al. 2010), biotic histories (Aronson et al. 2004), global climate change and marine diseases (Aronson and Precht 2006)].
Coral Reefs, 2009
We conducted a 20-week manipulative field experiment on shallow forereefs of the Florida Keys to assess the separate and interactive effects of herbivory and nutrient enrichment on the development of macroalgal communities and the fitness of the corals Porites porites and Siderastrea siderea. Excluding large herbivorous fishes produced macrophyte blooms both with and without nutrient enrichment. In contrast, there were no direct effects of nutrient enrichment. There were, however, small, but significant, interactive effects of herbivory and enrichment on macroalgal cover. Following nutrient enrichment, total macroalgae and the common seaweeds Dictyota spp. were suppressed in the presence, but not in the absence, of large herbivorous fishes-suggesting that fishes were selectively feeding on nutrient-enriched macrophytes. Access by large herbivores prevented algal overgrowth of corals, but these large fishes also directly grazed both corals. Excluding fishes did not alter survivorship of either coral species, but did decrease parrotfish grazing scars on both corals and increased the net growth of P. porites. Nutrient additions had no direct effects on the survivorship of corals, but there was a trend (P = 0.097) for nutrients to stimulate the growth of P. porites. The preponderance of experiments available to date indicates that loss of key herbivores is a major factor driving macroalgal blooms on coral reefs; anthropogenic nutrient pollution generally plays a more minor role.
Oecologia, 2012
Maintaining coral reef resilience against increasing anthropogenic disturbance is critical for effective reef management. Resilience is partially determined by how processes, such as herbivory and nutrient supply, affect coral recovery versus macroalgal proliferation following disturbances. However, the relative effects of herbivory versus nutrient enrichment on algal proliferation remain debated. Here, we manipulated herbivory and nutrients on a coral-dominated reef protected from fishing, and on an adjacent macroalgal-dominated reef subject to fishing and riverine discharge, over 152 days. On both reefs, herbivore exclusion increased total and upright macroalgal cover by 9–46 times, upright macroalgal biomass by 23–84 times, and cyanobacteria cover by 0–27 times, but decreased cover of encrusting coralline algae by 46–100% and short turf algae by 14–39%. In contrast, nutrient enrichment had no effect on algal proliferation, but suppressed cover of total macroalgae (by 33–42%) and cyanobacteria (by 71% on the protected reef) when herbivores were excluded. Herbivore exclusion, but not nutrient enrichment, also increased sediment accumulation, suggesting a strong link between herbivory, macroalgal growth, and sediment retention. Growth rates of the corals Porites cylindrica and Acropora millepora were 30–35% greater on the protected versus fished reef, but nutrient and herbivore manipulations within a site did not affect coral growth. Cumulatively, these data suggest that herbivory rather than eutrophication plays the dominant role in mediating macroalgal proliferation, that macroalgae trap sediments that may further suppress herbivory and enhance macroalgal dominance, and that corals are relatively resistant to damage from some macroalgae but are significantly impacted by ambient reef condition.
Marine Ecology Progress Series, 2003
We studied the effects of herbivory, fertilization and their interaction on algal succession on dead coral surfaces and the condition of live coral colonies. We used replicate open, closed, fertilized, and unfertilized cages in a 2-factor, 2-level design, sampled 7 times over a 49 d summer period at an offshore reef atoll lagoon in Belize. Herbivory negatively influenced algal biomass, whereas nutrients positively influenced wet but not dry or decalcified measures. Total and turf algal cover were positively influenced by nutrients and negatively by herbivory. Biomass was more strongly influenced by herbivory than fertilization, and the opposite was true for cover. Brown frondose algal cover was negatively influenced by both herbivory and nutrients, whereas red frondose algal cover was negatively affected by herbivory but unaffected by nutrients. There were more algal taxa and higher dominance in low compared to high herbivory treatments. In contrast to the relative dominance model (RDM), we found that turf algae did best under conditions of low herbivory and high nutrients, and also dominated high herbivory and low nutrient conditions, whereas frondose brown algae did best under low herbivory and low nutrient conditions, and appeared to be inhibited by high nutrients. Stony corals did sufficiently well in all conditions such that it was not possible to determine their optimal conditions, but elevated nutrients may provide resistance to end-of-summer bleaching by increasing the standing densities of algal symbionts. There were no detectable changes in symbiont community composition with all symbionts being members of Symbiodinium clade A. Coral mortality and low herbivory are most likely to be responsible for the high levels of brown frondose algae reported on these patch reefs.