Brian E. Lapointe

Nutrient Inputs from the Watershed and Coastal Eutrophication in the Florida Keys

Widespread use of septic tanks in the Florida Keys increase the nutrient concentrations of limestone groundwaters that discharge into shallow nearshore waters, resulting in coastal eutrophication. This study characterizes watershed nutrient inputs, transformations, and effects along a land-sea gradient stratified into four ecosystems that occur with increasing distance from land: manmade canal systems (receiving waters of nutrient inputs), seagrass meadows, patch reefs, and offshore bank reefs. Soluble reactive phosphorus (SRP), the primary limiting nutrient, was significantly elevated in canal systems compared to the other ecosystems, while dissolved inorganic nitrogen (DIN; NH4+ and NO3−) a secondary limiting nutrient, was elevated both in canal systems and seagrass meadows. SRP and NH4+ concentrations decreased to low concentrations within approximately 1 km and 3 km from land, respectively. DIN and SRP accounted for their greatest contribution (up to 30%) of total N and P pools in canals, compared to dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) that dominated (up to 68%) the total N and P pools at the offshore bank reefs. Particulate N and P fractions were also elevated (up to 48%) in canals and nearshore seagrass meadows, indicating rapid biological uptake of DIN and SRP into organic particles. Chlorophylla and turbidity were also elevated in canal systems and seagrass meadows; chlorophylla was maximal during summer when maximum watershed nutrient input occurs, whereas turbidity was maximal during winter due to seasonally maximum wind conditions and sediment resuspension. DO was negatively correlated with NH4+ and SRP; hypoxia (DO<2.5 mg l−1) frequently occurred in nutrient-enriched canal systems and seagrass meadows, especially during the warm summer months. These findings correlate with recent (<5 years) observations of increasing algal blooms, seagrass epiphytization and die-off, and loss of coral cover on patch and bank reef ecosystems, suggesting that nearshore waters of the Florida Keys have entered a stage of critical eutrophication.

Nutrient availability to marine macroalgae in siliciclastic versus carbonate-rich coastal waters

Abundant populations of frondose epilithic macroalgae from a variety of carbonate-rich tropical waters were significantly depleted in phosphorus relative to carbon and nitrogen when compared to macroalgae from temperate siliciclastic waters. Percent carbon (C) and percent nitrogen (N) dry weight contents were similar between tissues from the siliciclastic and carbonate environments (means of 22.6% vs. 20.1% and 1.0% vs. 1.2%, respectively), but phosphorus (P) levels were two-fold lower (0.15% vs. 0.07%) in the carbonate-rich systems. Accordingly, the molar C:N tissue ratios were comparable between macroalgae from the siliciclastic and carbonate sites (mean of 29.2 vs. 23.1), whereas large differences were observed for the C:P (mean of 430 vs. 976) and N:P ratios (mean of 14.9 vs. 43.4). In addition, alkaline phosphatase activity was low and often undetectable in the macroalgae from siliciclastic habitats (mean of 7.3 μM PO43− released g dry wt−1 h−1) compared to seven-fold higher rates (52.5 μM PO43− released g dry wt−1 h−1) observed in the macroalgae from carbonate systems. Seawater samples taken adjacent to benthic macroalgae from the carbonate-rich tropical waters contained relatively high levels of dissolved inorganic nitrogen with low concentrations of soluble reactive phosphorus (SRP), and showed elevated N:SRP ratios (mean=36) compared to siliciclastic environments (mean <3). These data support the precept that availability of N limits the productivity of macroalgae in temperate siliciclastic waters but, conversely, suggest that availability of P, rather than N, may be of paramount importance in limiting primary production of macroalgae in carbonate-rich tropical waters.

Effects of Stormwater Nutrient Discharges on Eutrophication Processes in Nearshore Waters of the Florida Keys

Rainfall events cause episodic discharges of groundwaters contaminated with septic tank effluent into nearshore waters of the Florida keys, enhancing eutrophication in sensitive coral reef communities. Our study characterized the effects of stormwater discharges by continuously (30-min intervals) measuring salinity, temperature, tidal stage, and dissolved oxygen (DO) along an offshore eutrophication gradient prior to and following heavy rainfall at the beginning of the 1992 rainy season. The gradient included stations at a developed canal system (PP) on Big Pine Key, a seagrass meadow in a tidal channel (PC), a nearshore patch reef (PR), a bank reef at Looe Key National Marine Sanctuary (LK), and a blue water station (BW) approximately 9 km off of Big PIne Key. Water samples were collected at weekly intervals during this period to determine concentrations of total nitrogen (TN), ammonium (NH4+), nitrate plus nitrite NO3− plus NO2−), total phosphorus (TP), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP), and chlorophyll a (chl a). Decreased salinity immediately followed the first major rainfall at Big Pine Key, which was followed by anoxia (DO <0.1 mg I−1), high concentrations of NH4+ (≈24 μM), TDP (≈1.5 μM), and chl a (≈20 μg I−1). Maximum concentration of TDP (≈0.30 μM) also followed the initial rainfall at the PC, PR, and LK stations. In contrast, NH4+ (≈4.0 μM) and chl a (0.45 μg I−1) lagged the rain event by 1–3 wk, depending on distance from shore. The highest and most variable concentrations of NH4+, TDP, and chl a occurred at PP, and all nutrient parameters correlated positively with rainfall. DO at all stations was positively correlated with tide and salinity and the lowest values occurred during low tide and low salinity (high rainfall) periods. Hypoxia (DO <2.5 mg I−1) was observed at all stations follwing the stormwater discharges, including the offshore bank reef station LK. Our study demonstrated that high frequency (daily) sampling is necessary to track the effects of episodic rainfall events on water quality and that such effects can be detected at considerable distances (12 km) from shore. The low levels of DO and high levels of nutrients and chl a in coastal waters of the Florida Keys demand that special precautions be exercised in the treatment and discharge of wastewaters and land-based runoff in order to preserve sensitive coral reef communities.

The relative importance of nutrient enrichment and herbivory on macroalgal communities near Norman's Pond Cay, Exumas Cays, Bahamas: a ''natural'' enrichment experiment

The simultaneous effects of grazing and nutrient enrichment on macroalgal communities were experimentally investigated using plastic mesh enclosure/exclosure cages along a natural nutrient (DIN, SRP) gradient from the discharge of a tidal mangrove creek on the west side of Norman’s Pond Cay, Exumas Cays, Bahamas. Nutrient enrichment was the only factor affecting total biomass of macroalgae whereas selective herbivory moderated species composition. Biomass ranged from >2 kg dry weight m 2 at the DIN-enriched Waterfall to <600 g dry weight m 2 in the DIN-limited Algal Halo. Grazing by the queen conch Strombus gigas and the sea hare Aplysia dactylomela significantly reduced biomass of the epiphyte Laurencia intricata, but not its host Digenea simplex at the DINenriched Waterfall. These two rhodophytes dominated the macroalgal community at the DIN-enriched Waterfall and inner Algal Halo, whereas the chlorophytes Microdictyon marinum and Cladophora catenata dominated the relatively DIN-depleted outer Algal Halo. Higher grazing rates by S. gigas, A. dactylomela, and juvenile parrotfish and surgeonfish at the DIN-enriched Waterfall compared to the more oligotrophic Algal Halo suggested selective grazing on higher quality (nitrogen) diets by these herbivores. A pronounced increase in the carbon/nitrogen (C:N) ratios of D. simplex and L. intricata from the Waterfall to the Algal Halo indicated increased DIN-limitation of growth in the more offshore, lower DIN macroalgal communities. In contrast, only modest increases in C:N ratios of M. marinum and C. catenata between the Waterfall and Algal Halo suggested that these species are better

Strategies for pulsed nutrient supply to Gracilaria cultures in the Florida Keys: Interactions between concentration and frequency of nutrient pulses☆

A factorial design experiment, using in situ cage cultures, was used to investigate the effects of frequency and concentration of nutrient pulses on growth, nutrient uptake, and chemical composition (C, N, P) of Gracilaria tikvahiae McLachlan in nearshore waters of the Florida Keys. Both frequency and concentration of the nutrient pulses affected growth and chemical composition of G. tikvahiae, indicating nutrient limitation occurred during the study. Growth of G. tikvahiae increased with increasing pulse frequency up to the highest level used (2·wk−1) at all pulse concentrations; in contrast, growth increased with increasing pulse concentration to the highest concentration at the low pulse frequency but not at the higher pulse frequencies. Although the frequency of nutrient pulses appeared more important in regulating growth and pre-pulse levels of chemical constituents than pulse concentration, the effects of frequency were due to its effects on total nutrient loading (i.e. flux) and not to the effects of frequency of nutrient enrichment per se. Greater variation in percent P compared to percent N in G. tikvahiae tissue between pulses and an increased PO3−4 uptake rate in nutrient-limited G. tikvahiae suggests that P rather than N was the primary limiting nutrient during the study; however, N was an important secondary limiting nutrient indicating dual nutrient-limitation occurred. While the pulse medium used had a N:P ratio of 18 : 1, much higher uptake ratios, ranging from 27:1 to 80:1, actually occurred, supporting the contention of P-limitation. Thus, nutrient pulse strategies with G. tikvahiae in P-limited systems need to utilize excessively lowN:P ratios in the pulse medium to offset the differential uptake rates of NH+4 and PO3−4 at the high concentrations typically used in pulse-feeding strategies.

A comparison of nutrient- and light-limited photosynthesis in psammophytic versus epilithic forms of Halimeda (Caulerpales, Halimedaceae) from the Bahamas

The relative nutritional status, with respect to phosphorus (Pi) vs. nitrogen (N) limitation, and light-limited photosynthesis (Ps) was examined over a broad range of quantum fluxes (I) for four Halimeda species, Halimeda tuna (Ellis and Solander) Lamouroux, H. simulans Howe, H. lacrimosa Howe and H. copiosa Goreau and Graham, taken from clear, shallow, Bahamian waters. The results support the hypothesis that psammophytic forms (i.e., sand dwellers anchored by a bulbous rhizoidal system) differ in nutrient status from epilithic forms (i.e., attached to rock by inconspicuous rhizoids). Maximum photosynthetic rates (Pmax) for the epilithic species H. lacrimos and H. copiosa decreased (P<0.05) following Pi enrichment, but increased (P<0.05) following N pulses. Conversely, following brief exposures to Pi, Pmax in the sand-dwelling forms H. tuna and H. simulans was elevated (P<0.05). These findings suggest that shallow species of Halimeda are adapted to take advantage of episodic nutrient pulses, and that partitioning of limiting resources may occur between the various life forms. Shallow water Halimeda species appear well adapted to variable light regimes, including low light conditions. In all cases, light-saturated photosyntheses (Ik) occurred at irradiances much lower than the ambient levels available on typical sunny days. Associated with low saturation irradiances were low light requirements for photosynthetic compensation (Ic) and reasonably efficient use of low photon flux densities as indicated by relatively steep slopes (α) of the Ps vs. I curves. Of the four species, H. copiosa was the most shade adapted, with considerably higher α values and considerably lower Ic, Ik and photoinhibition values.

The use of δ15N in assessing sewage stress on coral reefs.

While coral reefs decline, scientists argue, and effective strategies to manage land-based pollution lag behind the extent of the problem. There is need for objective, cost-effective, assessment methods. The measurement of stable nitrogen isotope ratios, delta(15)N, in tissues of reef organisms shows promise as an indicator of sewage stress. The choice of target organism will depend upon study purpose, availability, and other considerations such as conservation. Algae are usually plentiful and have been shown faithfully to track sewage input. The organic matrix of bivalve shells can provide time series spanning, perhaps, decades. Gorgonians have been shown to track sewage, and can provide records potentially centuries-long. In areas where baseline data are lacking, which is almost everywhere, delta(15)N in gorgonians can provide information on status and trends. In coral tissue, delta(15)N combined with insoluble residue determination can provide information on both sewage and sediment stress in areas lacking baseline data. In the developed world, delta(15)N provides objective assessment in a field complicated by conflicting opinions. Sample handling and processing are simple and analysis costs are low. This is a method deserving widespread application.

Land-based nutrient enrichment of the Buccoo Reef Complex and fringing coral reefs of Tobago, West Indies.

Tobagos fringing coral reefs (FR) and Buccoo Reef Complex (BRC) can be affected locally by wastewater and stormwater, and regionally by the Orinoco River. In 2001, seasonal effects of these inputs on water-column nutrients and phytoplankton (Chl a), macroalgal C:N:P and delta(15)N values, and biocover at FR and BRC sites were examined. Dissolved inorganic nitrogen (DIN, particularly ammonium) increased and soluble reactive phosphorus (SRP) decreased from the dry to wet season. Wet season satellite and Chl a data showed that Orinoco runoff reaching Tobago contained chromophoric dissolved organic matter (CDOM) but little Chl a, suggesting minimal riverine nutrient transport to Tobago. C:N ratios were lower (16 vs. 21) and macroalgal delta(15)N values higher (6.6 per thousand vs. 5.5 per thousand) in the BRC vs. FR, indicating relatively more wastewater N in the BRC. High macroalgae and low coral cover in the BRC further indicated that better wastewater treatment could improve the health of Tobagos coral reefs.

Reevaluation of ENCORE: support for the eutrophication threshold model for coral reefs.

Abstract The results from the multimillion dollar Enrichment of Nutrients on Coral Reefs Experiment (ENCORE) on One Tree Island Reef (OTIR) suggest that increased nutrient loads to coral reefs will have little or no effect on the algal growth rates and, hence, on the associated effects that increased algal growth might have on the functioning and stability of coral reefs. However, a comparison of the concentrations of nutrients within the OTIR lagoon with the proposed nutrient threshold concentrations (NTC) for coral reefs suggests that all sites, including the control sites, were saturated with nutrients during ENCORE, and, hence, one would not expect to get any differences between treatments in the algal-growth related measurements. Thus, ENCORE results provide strong support for the proposed NTCs and support the ecological principle that algal productivity and, consequently, the functioning of coral reefs are sensitive to small changes in the background concentrations of nutrients. The principal conclusion of ENCORE, namely that the addition of nutrients did not cause the “pristine” OTIR to convert from coral communities to algal dominated reefs, is contrary to the fact that there was prolific macroalgal growth on the walls and crests of the experimental microatolls by the end of ENCORE.

Satellite-Observed Black Water Events off Southwest Florida: Implications for Coral Reef Health in the Florida Keys National Marine Sanctuary

A “black water” event, as observed from satellites, occurred off southwest Florida in 2012. Satellite observations suggested that the event started in early January and ended in mid-April 2012. The black water patch formed off central west Florida and advected southward towards Florida Bay and the Florida Keys with the shelf circulation, which was confirmed by satellite-tracked surface drifter trajectories. Compared with a previous black water event in 2002, the 2012 event was weaker in terms of spatial and temporal coverage. An in situ survey indicated that the 2012 black water patch contained toxic K. brevis and had relatively low CDOM (colored dissolved organic matter) and turbidity but high chlorophyll-a concentrations, while salinity was somewhat high compared with historical values. Further analysis revealed that the 2012 black water was formed by the K. brevis bloom initiated off central west Florida in late September 2011, while river runoff, Trichodesmium and possibly submarine groundwater discharge also played important roles in its formation. Black water patches can affect benthic coral reef communities by decreasing light availability at the bottom, and enhanced nutrient concentrations from black water patches support massive macroalgae growth that can overgrow coral reefs. It is thus important to continue the integrated observations where satellites provide synoptic and repeated observations of such adverse water quality events.