Caribbean Journal of Science, Vol. 45, No. 2-3, 168-180, 2009 Copyright 2009 College of Arts and Sciences University of Puerto Rico, Mayagüez
Spatial and temporal patterns of water quality indicators in reef systems of southwestern Puerto Rico Ernesto Otero Department of Marine Sciences, P.O. Box 9013, University of Puerto Rico. Mayagüez, Puerto Rico 00681 Corresponding author e-mail:
[email protected]; to whom reprint requests should be addressed. Telephone: 1-787-899-2048 X 269, Facsimile: 1-787-899-5500 Running Title: Coral reef water quality Puerto Rico ABSTRACT.—Baseline information on the water quality status of coral reefs and associated habitats of La Parguera, southwestern PR, is presented. Measurements of temperature, salinity, chlorophyll a, turbidity, fluorescence of dissolved organic matter, bacterial abundance and productivity, bacteriophytoplankton, sedimentation and stable isotopes of particulate organic matter were made at various stations within the area from 2003 through 2005. Significant daily fluctuations were observed at all stations and seasonal changes indicate inputs of materials from terrestrial sources even at the shelf edge. The δ13C data indicates low inputs from terrestrial particles (min = -18 ‰ vs. PDB) while variations of dissolved organic matter fluorescence suggest periodic inputs of terrestrially derived matter and nutrients throughout the shelf, though most notable towards the coast. Chlorophyll a (mostly > 0.5mg.l-1), bacteria (ca. 1 x106 cells.ml-1) and bacteriophytoplankton (72-122 x103 cell.ml-1) indicate the presence of significant nutrient inputs while particle δ15N shows minimum to moderate enrichment from anthropogenic sources (max = 3.5 ‰ vs. air). Sedimentation rates were mostly 500g.m-2 of sediments were deposited over a period of 24 hours. The average sedimentation at Romero excluding the extreme episode
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(October 2005) was 90 g.m-2.d-1, the highest rate for all stations. Multiple regression analysis showed significant covariation between sedimentation rates as a function of turbidity and depth where log sed = 1.29 log NTU + 0.6 log Z + 1.10 (r2=0.68), where sed is sedimentation rate, NTU is turbidity and Z is station depth. Stable isotope signatures Particles collected with sediments traps showed a narrow range of δ13C signatures (-19.6 to -15.3o/oo; Table 1). Slightly depleted δ13C signatures were found close to the coast and at Media Luna reef, suggesting that the terrestrial particulate organic carbon inputs, such as that derived from mangroves and other woody vegetation, is very limited. The δ15N from sediment traps ranged from +1.4 at the shelf edge to slightly enriched values of >3.4 at Romero, Magueyes, and Varadero stations (Table 1). These low values suggest moderate to minimal inputs of anthropogenic nitrogen to local marine systems since nitrogen from sewage, septic seepage and agricultural fertilizers is, in contrast, highly enriched (Macko and Ostrom, 1994; McClelland et al. 1997; Aravena et al. 1993; Lapointe et al. 2004). Analysis of possible endpoints conducted for this study showed δ13C for algae, turtle grass, and red mangrove to be -15, -10 and -30 o/oo, respectively. The δ15N of mangrove and microalgae subjected to known sources of sewage within the study area was approximately +15 o/oo. Dissolved Organic Matter Fluorescence The spatial distribution of mean DOMFl was characterized by a sharp decrease towards the open ocean. The coastal stations with the highest d15N (Magueyes and Varadero) (Table 1) had the highest DOMFl while the inner reef stations showed a marked decrease to 2-3 ng QS.ml-1. The DOMFl revealed large temporal variation (3-4 ng QS.ml-1) at the mid and western coastal stations, which are closer to the developed portion of the coast and subjected to higher concentrations of terrestrial DOM during episodic rain events. The second highest levels of DOMFl corresponded
to Romero, Enrique and Pelotas reefs with values ranging from 2.55 and 2.83. The remaining reef stations had similar or lower DOMFl. Microbial variables.—Bacterial abundance showed a gradual southerly decrease from 1-2 x 106 close to the coast to 0.5 x 106cells. ml-1 at the shelf edge (Table 2). Bacterial production reached ca. 30 μgC.l-1.d-1 at stations where DOMFl was maximum and decreased to half that level at Matei, Romero and Náutico stations. Other stations shared similarly low values (7-9 ugC.l-1.d-1) while the shelf edge was approximately 10% as productive as most of the coastal stations. The range of bacteriophytoplankton was approximately 10% that of bacteria, ranging from 0.24-1.2 x 105 cells.ml-1, being maximum at the protected Pelotas reef. Discussion Physical measurements Coral bleaching and mortality are related to decreases in salinity, especially when salinity drops to 24 hrs (Lirman and Manzello, 2009). Relatively stable salinities were observed at most coral reef stations. However, Romero received significant amounts of fresh water related to a storm event that lowered the salinity at the base of the front reef to 100 NTU during severe weather conditions at the Great Barrier Reef (Orpin et al. 2004). Turbidities during the sampling period herein, reached minimum levels at the Shelf Edge, averaged 1 NTU at the other coral reef stations and 2 NTU at most of the coastal stations , well within the range observed by Loya (1976) and Lapointe and Clark (1992). Maximal values, from 5-10 NTU throughout the study area and > 100NTU in coastal stations were related to storms. The observed turbidity fluctuations indicate the frequency with which La Parguera reefs are subject to
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events that modify light penetration, sedimentation (see below) and the interaction of microbial biota with macrobenthos (Rogers, 1990; Fabricius, 2005; Oliver et al., 1983). Average sedimentation in La Parguera is typically moderate but with extreme spikes. Sedimentation of 100 g.m-2.d-1 can be considered stressful to coral species (Rogers, 1990). Corals subjected to sedimentation rates of 100-140 g.m-2.d-1 show signs of temperature stress while none was observed on corals under lower sedimentation rates (Nemeth and Sladek-Nowlis, 2001). While sedimentation rates in La Parguera were mostly moderate, a possible exception was observed at Romero reef, with an average of > 200 g.m-2.d-1. The higher values were influenced by high sediment resuspension and to smaller degree by run-off related to Hurricane Jeanne. Excluding that extreme event, sedimentation levels at Romero fall to moderately high levels similar to those of Enrique reef, suggesting that corals are, at most, moderately affected by sedimentation in present day reefs of La Parguera. The significant relationship found between turbidity and sedimentation provides a tool to estimate possible sedimentation rates. Turbidity measurements based on formazin standards are prone to underestimate light transmission at higher turbidities (i.e. >20NTU) according to Telesnicki and Goldberg (1995). This may introduce uncertainties in highly turbid habitats. However, since turbidity levels were mostly +3 ‰ were indicative of a significant subsidy by anthropogenic N sources in the Florida Keys, concluding that such nutrients impact a wide area of Florida′s reef track. A more conservative interpretation is given here to the slightly enriched δ15N found in La Parguera. The inherent complexities of N cycling, may obscure the 15N isotopic signature of different sources (Lehman et al. 2002) specially considering the low enrichment of material observed during this study. Furthermore, even samples collected close to known sources of anthropogenic N evidenced limited enrichment. The estimate of the proportion of N allocated from anthropogenic sources (%Na) in particles from sediment traps, was 3% furthest from the coast, 12-15% at mid-shelf stations, and approximately 20% close to the coast. Coastal sites subjected to increased flushing (Náutico and Ramp stations) have lower %Na. Interestingly, the %Na at Romero and Enrique reefs were as high as coastal values. The elevated signatures of mangrove and microalgal material used as δ15N
REEF WATER QUALITY IN SW PUERTO RICO
endpoints from sewage-impacted areas within La Parguera indicate that such inputs are present but rapidly diluted by those from other sources via mineralization or biogeochemical processes. The overall distribution of δ15N in trapped particles suggests that, at a system level, distinctly anthropogenic sources of N, at most, reach modest levels. The overall distribution of δ15N agrees with the efficient dispersal and distribution of potential contaminants from the town of La Parguera, including its sewage treatment facility. The intermediate values found at reef stations are indicative of shifts caused by mineralization processes due to in situ microbial processes but do not rule out inputs of terrigenous or anthropogenic sources of nutrients. In contrast, the stable isotopic evidence indicates that the dominant persistent source of N at the shelf edge is N-fixation. Planktonic processes Chlorophyll a concentrations in coral reef areas are usually
