Jorge E. Corredor

Analysis of the optical properties of the Orinoco River plume by absorption and fluorescence spectroscopy

The discharge of the Orinoco River significantly affects the optical properties of the water in the Caribbean Sea by increasing primary productivity and introducing large amounts of colored dissolved organic matter (CDOM) to the region. The optical characteristics of the CDOM in the Orinoco River plume were investigated during two cruises to the eastern Caribbean using absorption and fluorescence spectroscopy. We found that high concentrations of dissolved organic carbon (DOC) and abundance of CDOM in the eastern Caribbean were related to the Orinoco River plume. We did not observe changes in optical properties of CDOM in waters with salinities lower than 30. Self-shading from sunlight and the preponderance of riverine material at these low salinities can explain the observed behavior. Changes in the optical properties of the CDOM were observed in stations with salinities higher than 30 and were related to mixing between riverine and marine end-members, and to photodegradation. Light penetration and, consequently, the position of the chlorophyll maxima in areas close to the Gulf of Paria were controlled by the concentration of CDOM. Of the two main fluorophores found in this study, only the humic fluorophore appeared to be related to the absorption of light by CDOM in the visible spectrum. A secondary fluorophore, with excitation and emission in the UV, appeared to be resistant to photodegradation and did not show any relationship with absorbance in the visible range.

The impact of accelerating land-use change on the N-cycle of tropical aquatic ecosystems: Current conditions and projected changes

Published data and analyses from temperate and tropical aquatic systems are used to summarize knowledge about the potential impact of land-use alteration on the nitrogen biogeochemistry of tropical aquatic ecosystems, identify important patterns and recommend key needs for research. The tropical N-cycle is traced from pre-disturbance conditions through the phases of disturbance, highlighting major differences between tropical and temperate systems that might influence development strategies in the tropics. Analyses suggest that tropical freshwaters are more frequently N-limited than temperate zones, while tropical marine systems may show more frequent P limitation. These analyses indicate that disturbances to pristine tropical lands will lead to greatly increased primary production in freshwaters and large changes in tropical freshwater communities. Increased freshwater nutrient flux will also lead to an expansion of the high production, Nand light-limited zones around river deltas, a switch from Pto N-limitation in calcareous marine systems, with large changes in the community composition of fragile mangrove and reef systems. Key information gaps are highlighted, including data on mechanisms of nutrient transport and atmospheric deposition in the tropics, nutrient and material retention capacities of tropical impoundments, and N/P coupling and stoichiometric impacts of nutrient supplies on tropical aquatic communities. The current base of biogeochemical data suggests that alterations in the

Atmospheric Nitrous Oxide Fluxes from Mangrove Sediments

Abstract Although the marine environment is recognized as a net source of nitrous oxide to the atmosphere, current global budgets appear to underestimate the marine source of this radiatively active gas. Mangrove forests occupy a significant fraction of the tropical land/sea interface and receive high inputs of terrestrial nitrogen. Anthropogenic activities increase nitrogen input to mangrove sediments thereby enhancing microbial nitrogen metabolism. Elevated rates of nitrous oxide flux might therefore be expected. Our experimental determinations of nitrous oxide flux from mangrove sediments yield rates of between 0.12 and 7.8 μmol N2O m−2 h−1; values which are between 2 and 135 times greater than those reported previously for intertidal estuarine sediments. Nitrous oxide yield from these sediments varies greatly in response to both the magnitude and chemical nature of the sediment nitrogen source. Rates appear to be greater when reduced nitrogen is the dominant substrate and nitrification is the main source of nitrous oxide compared to sediments where nitrate and nitrite predominate and nitrous oxide arises from microbial denitrification.

Petroleum pollution and mutation in mangroves

Abstract Chlorophyll-deficiency has often been used as a sensitive genetic end-point in plant mutation research. The frequency of trees heterozygous for nuclear chlorophyll-deficient mutations was determined for mangrove populations growing along the southwest coast of Puerto Rico. The frequency of heterozygotes was strongly correlated with the concentration of polycyclic aromatic hydrocarbons in the underlying sediment and with both acute and chronic petroleum pollution. Although epidemiological studies can seldom prove causation, a strong correlation is certainly compatible with a cause-effect relationship. Our results suggest that the biota of oil-polluted habitats may be experiencing increased mutation.

Nitrate Depuration of Secondary Sewage Effluents in Mangrove Sediments

The sewage treatment plant (STP) at La Parguera, on the southwest coast of Puerto Rico, discharges an average of 228,000 dm3 of secondary sewage effluents per day into percolation ponds located at the landward margin of the coastal mangrove fringe. Effluents flowing from the STP percolation ponds to the adjacent mangrove fringe typically exhibited nitrate levels between 0.2 mM and 1.0 mM. Experimental determination of actual and potential denitrification using acetylene block and substrate disappearance techniques indicate that mangrove sediment microbial communities are capable of depurating 10 to 15 times the nitrate added in the STP effluent. Plots of porewater salinities vs nitrate concentrations show exponential decay of nitrate concentration. Our observations confirm the potential of mangrove sediment-microbial communities for nitrate depuration of secondary sewage effluents.

Nitrogen cycling and anthropogenic impact in the tropical interamerican seas

We discuss the mechanisms leading to nutrient limitation in tropical marine systems, with particular emphasis on nitrogen cycling in Caribbean ecosystems. We then explore how accelerated nutrient cycling from human activities is affecting these systems.

Methane Flux From Mangrove Sediments Along the Southwestern Coast of Puerto Rico

Although the sediments of coastal marine mangrove forests have been considered a minor source of atmospheric methane, these estimates have been based on sparse data from similar areas. We have gathered evidence that shows that external nutrient and freshwater loading in mangrove sediments may have a significant effect on methane flux. Experiments were performed to examine methane fluxes from anaerobic sediments in a mangrove forest subjected to secondary sewage effluents on the southwestern coast of Puerto Rico. Emission rates were measured in situ using a static chamber technique, and subsequent laboratory analysis of samples was by gas chromatography using a flame ionization detector. Results indicate that methane flux rates were lowest at the landward fringe nearest to the effluent discharge, higher in the seaward fringe occupied by red mangroves, and highest in the transition zone between black and red mangrove communities, with average values of 4 mg CH4 m−2 d−1, 42 mg CH4 m−2 d−1, and 82 mg CH4 m−2 d−1, respectively. Overall mean values show these sediments may emit as much as 40 times more methane than unimpacted pristine areas. Pneumatophores ofAviciennia germinans have been found to serve as conduits to the atmosphere for this gas. Fluctuating water level overlying the mangrove sediment is an important environmental factor controlling seasonal and interannual CH4 flux variations. Environmental controls such as freshwater inputs and increased nutrient loading influence in situ methane emissions from these environments.

Increase of nitrous oxide flux to the atmosphere upon nitrogen addition to red mangroves sediments

Response of nitrous oxide N20 sediment/air flux to nitrogen addition was assessed in mangrove (Rhizophora mangle) sediments. Fluxes were enhanced with both ammonium and nitrate loading. Greatest fluxes (52 micromol m(-2) h(-1)) were obtained with ammonium addition and saturation was achieved with additions of 0.9 mol m(-2). Maximum flux following ammonium addition was 2785 times greater than control plots and 4.5 times greater during low tide than with equivalent ammonium addition at high tide. Nitrate enrichment resulted in exponential growth, with maximal mean flux of 36.7 micromolm(-2) h(-1) at 1.9 molm(-2); saturation was not achieved. Differential response to ammonium and nitrate, and to tide and elevation, indicate that microbial nitrification is responsible for most of the observed gas flux. Mangrove sediments constitute an important source of global atmospheric N20 and increases in nitrogen loading will lead to significant increases in the flux of this atmospherically active gas.

Seasonal variation of physical and biogeochemical features in eastern Caribbean Surface Water

We describe vertical and temporal variation of water mass structure, dissolved nutrients, phytoplankton chlorophyll a (Chl a), and dissolved organic matter south of the island of Puerto Rico at 67°W, 17°36′N with particular emphasis on variability of Caribbean Surface Water (CSW) features. CSW at the Caribbean Time Series (CaTS) station experiences yearlong continental riverine influence as evidenced by surface salinity consistently below that of Tropical Atlantic Surface Water and elevated silicate content. Salinity exhibits close correlation to average rainfall over the Orinoco River basin with a 3–4 month offset, and salinity and silicate are in turn closely correlated. Calculations using a simple mixing model and based on known end-members for salinity and silicate indicate that rainwater accounts for 0–3.1% of CSW, while river water accounts for 0.6–5.5% at CaTS, indicating that surface waters of the NE Caribbean are under the influence of riverine water throughout the year. No significant correlation was found between salinity and phytoplankton Chl a, but the depth of the Chl a maximum was statistically correlated to seasonal salinity fluctuations. This observation may account for the apparent seasonal Chl a increase in these waters deduced from remote-sensing estimates. Analysis of historical data shows that seasonal shoaling of the Chl a maximum results in a net increase in phytoplankton carbon biomass during the period of influence of the Orinoco River plume.

Persistence of spilled crude oil in a tropical intertidal environment

Abstract Intertidal sediments at Bahia Sucia, on the southwest coast of Puerto Rico contain discreet subsurface layers of petrogenic hydrocarbons. The uppermost of these layers contains petroleum contaminants in concentrations above 200 mg g −1 and is most probably atributable to the 1977 spill from the tanker Zoe Colocotronis . A deeper layer containing a less concentrated load of petroleum hydrocarbons may correspond to the Argea Prima spill of 1962. Sediments above, between and below these two layers present low concentrations of typically biogenic hydrocarbons. Although petroleum released to the sea in tropical environments generally suffers rapid degradation, petroleum contaminants reaching intertidal sediments may exhibit long-term persistence.