Nutrient thresholds for bottom‐up control of macroalgal blooms on coral reefs in Jamaica and southeast Florida

Abstract

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.

Linking regional sources and pathways for submarine groundwater discharge at a reef by electrical resistivity tomography, 222Rn, and salinity measurements

“…These studies show that SGD delivers substantial amounts of nutrients to reefs thereby affecting reef ecology and biogeochemistry [Paytan et al, 2006;Street et al, 2008]. Since SGD is potentially a critical natural component of coral reefs [e.g., D'Elia et al, 1981;Lapointe, 1997], its role needs to be fully understood in light of the gloomy prospect for coral reefs under combined natural and human-induced global change [Bellwood et al, 2004;Carpenter et al, 2008].…”

Ecological Shifts along the Florida Reef Tract: The Past as a Key to the Future

“…In general, the same maladies affecting coral reefs in Florida are present throughout the Caribbean, with reefs both near and far from anthropogenic sources of nutrients displaying similar community shifts from coral to macroalgal domination (Aronson and Precht 2001c). For instance, Lapointe, Littler, and Littler (1997) used reefs of the offshore barrier reef tract of Belize in the vicinity of Carrie Bow Cay as an example of a low-nutrient (oligotrophic) reef system and as a counterpoint to the Florida reef tract (Lapointe 1997). Like Florida, these Belizean reefs have recorded significantly reduced coral populations, especially the acroporids, with concomitant increases in macroalgae over the past two decades in the absence of nutrient enrichment (Rützler and Macintyre 1982;Littler et al 1987;Aronson et al 1994;McClanahan and Muthiga 1998;McClanahan et al 1999;Aronson and Precht 2001b;.…”

“…In Florida, this implied failure is based on the hypothesis that diminished water quality is the single most important cause of environmental degradation in the Everglades, in Florida Bay, and on the reefs of the Florida Keys, and that the woes that beset these systems are local, human-induced, and reversible. It is easy to come away with this view after reading the literature (Lapointe 1997;Bacchus 2002;Brand 2002;Lapointe, Matzie, and Barile 2002;Lipp et al 2002;Patterson et al 2002;Porter et al 2002) and being exposed to the idea as presented in the popular media (e.g., Lapointe 1989;Torrance 1991;Dustan 1997;Barnett 2003), but evidence linking nutrient loading to reef degradation remains elusive (Szmant 2002).…”

“…Distinctions are typically not made between benthic habitats nearshore (especially hardbottom communities and seagrass beds close to shore) and the main reef tract located 5 to 7 km offshore. While few data exist to suggest that changes in offshore reef communities were caused by local pollution sources in Florida, and although few historical data exist regarding nutrient pollution, algal abundance, or coral cover (Szmant 2002), strenuous debate about the relative importance of topdown versus bottom-up (Lapointe 1997(Lapointe , 1999 processes continues (see Davidson 1998).…”

Tidal variability of nutrients in a coastal coral reef system influenced by groundwater

“…In addition, groundwater tidally driven into nearshore ecosystems was found to be negatively correlated with seagrass habitat conditions (Houk et al, 2013). Nutrient loads via groundwater discharge may affect the community structure to move towards macroalgal blooms via bottom-up control (Lapointe, 1997) and likely play a role in the displacement of slow-growing benthic flora with fast-growing species observed in Sanya Bay in the last two decades (Titlyanov et al, 2015). Future changes in these fluxes, likely caused by climate change and human activities, might make the situation worse and need to be monitored in reef protection programs and be considered in assessing the environmental health of coral reef systems, especially in regions with expected higher inputs of anthropogenic nutrients into the groundwater.…”