The exchange of carbon, nitrogen, and phosphorus in dwarf and fringe mangroves of the oligotrophic southern Everglades

Water management has altered both the natural timing and volume of freshwater delivered to Everglades National Park. This is especially true for Taylor Slough and the C-111 basin, as hypersaline events in Florida Bay have been linked to reduced freshwater flow in this area. In light of recent efforts to restore historical flows to the eastern Everglades, an understanding of the impact of this hydrologic shift is needed in order to predict the trajectory of restoration. I conducted a study to assess the importance of season, water chemistry, and hydrologic conditions on the exchange of nutrients in dwarf and fringe mangrove wetlands along Taylor Slough. I also performed mangrove leaf decomposition studies to determine the contribution of biotic and abiotic processes to mass loss, the effect of salinity and season on degradation rates, and the importance of this litter component as a rapid source of nutrients. ^ Dwarf mangrove wetlands consistently imported total nutrients (C, N, and P) and released NO2− +NO3 −, with enhanced release during the dry season. Ammonium flux shifted from uptake to release over the study period. Dissolved phosphate activity was difficult to discern in either wetland, as concentrations were often below detection limits. Fluxes of dissolved inorganic nitrogen in the fringe wetland were positively related to DIN concentrations. The opposite was found for total nitrogen in the fringe wetland. A dynamic budget revealed a net annual export of TN to Florida Bay that was highest during the wet season. Simulated increases and decreases in freshwater flow yielded reduced exports of TN to Florida Bay as a result of changes in subsystem and water flux characteristics. Finally, abiotic processes yielded substantial nutrient and mass losses from senesced leaves with little influence of salinity. Dwarf mangrove leaf litter appeared to be a considerable source of nutrients to the water column of this highly oligotrophic wetland. To summarize, nutrient dynamics at the subsystem level were sensitive to short-term changes in hydrologic and seasonal conditions. These findings suggest that increased freshwater flow has the potential to lead to long-term, system-level changes that may reach as far as eastern Florida Bay. ^

Nutrient content of the seagrass Thalassia testudinum reveals regional patterns of relative availability of nitrogen and phosphorus in the Florida Keys USA

Between 1992 and 2000, we sampled 504 randomly chosen locations in theFlorida Keys, Florida, USA, for the elemental content of green leaves of theseagrass Thalassia testudinum. Carbon content ranged from29.4–43.3% (dry weight), nitrogen content from 0.88–3.96%, andphosphorus content from 0.048–0.243%. N and P content of the samples werenot correlated, suggesting that the relative availability of N and P variedacross the sampling region. Spatial pattern in C:N indicated a decrease in Navailability from inshore waters to the reef tract 10 km offshore;in contrast, the pattern in C:P indicated an increase in P availability frominshore waters to the reef tract. The spatial pattern in N:P was used to definea P-limited region of seagrass beds in Florida Bay and near shore, and anN-limited region of seagrass beds offshore. The close juxtaposition ofN–and P-limited regions allows the possibility that N loading from thesuburban Florida Keys could influence the offshore, N-limited seagrass bedswithout impacting the more nearshore, P-limited seagrass beds. Carbonate - Nutrient lim

Responses of seagrass communities to fertilization along a gradient of relative availability of nitrogen and phosphorus in a carbonate environment

Patterns of relative nutrient availability in south Florida suggest spatial differences regarding the importance of nitrogen (N) and phosphorus (P) to benthic primary producers. We did a 14-month in situ fertilization experiment to test predictions of N and P limitation in the subtropical nearshore marine waters of the upper Florida Keys. Six sites were divided into two groups (nearshore, offshore) representing the endpoints of an N: P stoichiometric gradient. Twenty-four plots were established at each site with six replicates of each treatment (1N, 1P, 1N1P, control), for a total of 144 experimental plots. The responses of benthic communities to N and P enrichment varied appreciably between nearshore and offshore habitats. Offshore seagrass beds were strongly limited by nitrogen, and nearshore beds were affected by nitrogen and phosphorus. Nutrient addition at offshore sites increased the length and aboveground standing crop of the two seagrasses, Thalassia testudinum and Syringodium filiforme, and growth rates of T. testudinum. Nutrient addition at nearshore sites increased the relative abundance of macroalgae, epiphytes, and sediment microalgae. N limitation of seagrass in this carbonate system was clearly demonstrated. However, added phosphorus was retained in the system more effectively than N, suggesting that phosphorus might have important long-term effects on these benthic communities. The observed species-specific responses to nutrient enrichment underscores the need to monitor all primary producers when addressing questions of nutrient limitation and eutrophication in seagrass communities.

Benthic Exchange of C, N, and P Along the Estuarine Ecotone of Lower Taylor Slough, Florida (USA): Effect of Seasonal Flows and Phosphorus Availability

The southern Everglades and Florida Bay have experienced a nearly 50 % reduction in freshwater flow resulting in increased salinity and landward expansion of mangrove forest. Given the marine end-member is a natural source of P to this region, it is necessary to understand the interactions between inflows and P availability in controlling the exchange of materials across the mangrove ecotone. From 2007 to 2008, we used sediment core incubations to quantify fluxes of dissolved inorganic N and P and dissolved organic carbon (DOC) in three ecotone areas (dwarf mangrove, pond, and bay). Experiments were repeated seasonally over 2 years involving P-enriched surface water as a factor. We saw consistent uptake of soluble reactive P (SRP), DOC, and nitrate + nitrite (N+N) by the soils/sediments and release of ammonium (NH4 +) from soils/sediments to the water column across all sites and seasons. P enrichment had no discernible effect on DIN or DOC flux, suggesting that rapid P uptake may have been more geochemically mediated. However, uptake of added P occurred across all sites and seasons, reflecting high uptake capacity in this carbonate system and the potential of the mangrove ecotone to sequester P as it becomes more available.

Carbon (d13c) and Nitrogen (d15n) Isotopic Discrimination in Mangroves in Florida Coastal Everglades as a Function of Environmental Stress

Isotope signatures of mangrove leaves can vary depending on discrimination associated with plant response to environmental stressors defined by gra­dients of resources (such as water and nutrient limitation) and regulators (such as salinity and sul­fide toxicity). We tested the variability of man­grove isotopic signatures (d13C and d15N) across a stress gradient in south Florida, using green leaves from four mangrove species collected at six sites. Mangroves across the landscape studied are stressed by resource and regulator gradients repre­sented by limited phosphorus concentrations com­bined with high sulfide concentrations, respec­tively. Foliar d13C ratios exhibited a range from ­ 24.6 to –32.7‰, and multiple regression analysis showed that 46% of the variability in mangrove d13C composition could be explained by the differ­ences in dissolved inorganic nitrogen, soluble reac­tive phosphorus, and sulfide porewater concentra­tions. 15N discrimination in mangrove species ranged from –0.1 to 7.7‰, and porewater N, salin­ity, and leaf N:Pa ratios accounted for 41% of this variability in mangrove leaves. The increase in soil P availability reduced 15N discrimination due to higher N demand. Scrub mangroves (<1.5 m tall) are more water-use efficient, as indicated by higher d13C; and have greater nutrient use efficiency ratios of P than do tall mangroves (5 to 10 m tall) existing in sites with greater soil P concentrations. The high variability of mangrove d13C and d15N across these resource and regulator gradients could be a con­founding factor obscuring the linkages between mangrove wetlands and estuarine food webs. These results support the hypothesis that landscape fac­tors may control mangrove structure and function, so that nutrient biogeochemistry and mangrove-based food webs in adjacent estuaries should ac­count for watershed-specific organic inputs.

Quantifying nitrogen cycling rates in freshwater marshes of the southern Everglades using nitrogen-15 tracer techniques

This dissertation research project addressed the question of how hydrologic restoration of the Everglades is impacting the nutrient dynamics of marsh ecosystems in the southern Everglades. These effects were analyzed by quantifying nitrogen (N) cycle dynamics in the region. I utilized stable isotope tracer techniques to investigate nitrogen uptake and cycling between the major ecosystem components of the freshwater marsh system. I recorded the natural isotopic signatures (δ15N and δ 13C) for major ecosystem components from the three major watersheds of the Everglades: Shark River Slough, Taylor Slough, and C-111 basin. Analysis of δ15 N and δ13C natural abundance data were used to demonstrate the spatial extent to which nitrogen from anthropogenic or naturally enriched sources is entering the marshes of the Everglades. In addition, I measured the fluxes on N between various ecosystem components at both near-canal and estuarine ecotone locations. Lastly, I investigated the effect of three phosphorus load treatments (0.00 mg P m-2, 6.66 mg P m-2, and 66.6 mg P m-2) on the rate and magnitude of ecosystem N-uptake and N-cycling. The δ15N and δ13C natural abundance data supported the hypothesis that ecosystem components from near-canal sites have heavier, more enriched δ 15N isotopic signatures than downstream sites. The natural abundance data also showed that the marshes of the southern Everglades are acting as a sink for isotopically heavier, canal-borne dissolved inorganic nitrogen (DIN) and a source for "new" marsh derived dissolved organic nitrogen (DON). In addition, the 15N mesocosm data showed the rapid assimilation of the 15N tracer by the periphyton component and the delayed N uptake by soil and macrophyte components in the southern Everglades.

Mangrove tannins in aquatic ecosystems: Their fate and possible influence on dissolved organic carbon and nitrogen cycling

We describe the fate of mangrove leaf tannins in aquatic ecosystems and their possible influence on dissolved organic nitrogen (DON) cycling. Tannins were extracted and purified from senescent yellow leaves of the red mangrove (Rhizophora mangle) and used for a series of model experiments to investigate their physical and chemical reactivity in natural environments. Physical processes investigated included aggregation, adsorption to organic matter-rich sediments, and co-aggregation with DON in natural waters. Chemical reactions included structural change, which was determined by excitation–emission matrix fluorescence spectra, and the release of proteins from tannin–protein complexes under solar-simulated light exposure. A large portion of tannins can be physically eliminated from aquatic environments by precipitation in saline water and also by binding to sediments. A portion of DON in natural water can coprecipitate with tannins, indicating that mangrove swamps can influence DON cycling in estuarine environments. The chemical reactivity of tannins in natural waters was also very high, with a half-life of less than 1 d. Proteins were released gradually from tannin–protein complexes incubated under light conditions but not under dark conditions, indicating a potentially buffering role of tannin– protein complexes on DON recycling in mangrove estuaries. Although tannins are not detected at a significant level in natural waters, they play an important ecological role by preserving nitrogen and buffering its cycling in estuarine ecosystems through the prevention of rapid DON export/loss from mangrove fringe areas and/or from rapid microbial mineralization.

Patterns of phosphorus, nitrogen and δ15N along a peat development gradient in a coastal mire, Panama

Differentiation of limiting nutrients within small spatial scales has been observed in coastal mangrove forests, but research on other tropical peatlands suggests it is a more widespread phenomenon. In the Changuinola mire of coastal Panama, oligotrophy was hypothesized to increase along a gradient of peat development (peat doming). Nutrient and carbon concentration of leaf tissue, soil, and soil porewater were characterised over a successive sequence of plant communities along the gradient. Soil phosphorus (P) and nitrogen (N) concentrations decreased from 1200 μg P g−1 and 27 mg N g−1 to 377 μg P g−1 and 22 mg N g−1 within 2.7 km into the mire interior. These changes coincided with an increase in soil and average leaf N:P molar ratios from 52–128 and 24–41, respectively. Soil P was strongly related to leaf P and soil N:P to foliar N:P. There was a wide range in δ15N values for canopy (4.0 to −9.4‰), Campnosperma panamense (4.0 to −7.8‰) and understorey (4.8 to −3.1‰) species. Foliar δ15N values of canopy species were strongly related to soil N:P, soil P and leaf P. The depleted foliar δ15N values appeared to be an effect of both the N atmospheric source and P limitation. Here, P limitation is likely associated with ombrotrophic conditions that developed as hydrologic inputs became dominated by precipitation.

Coral disease dynamics and environmental drivers in the northern Florida Keys and Lee Stocking Island, Bahamas

Coral reefs are experiencing declines worldwide and recently coral diseases have been identified as significant contributors to coral mortality. However, little is known regarding the factors that drive coral disease distributions and dynamics. Current knowledge of the organisms that cause coral diseases is also limited, with pathogens having been identified for only 5 of the 21 described coral diseases. The study presented here describes coral disease dynamics in terms of occurrence, prevalence, spatial distribution, and host species susceptibility from 2002--2004 on reefs of the Northern Florida Keys (NFK) and Lee Stocking Island (LSI) in the Bahamas' Exuma chain. In addition, this research investigated the influence of temperature, sediment, and nutrient availability on coral disease prevalence and severity. Finally, microbial communities associated with a polymicrobial disease, black band, were examined to address spatial and temporal variability. ^ Four scleractinian diseases were observed in repeated surveys conducted during June-August of each year: black band disease (BBD), white plague type 2 (WP), dark spots syndrome (DSS), and yellow band disease-(YBD). Coral disease prevalence was generally low in both the NFK and LSI as compared to epizootic levels reported previously in the NFK and other regions of the Caribbean. Disease prevalence and species susceptibility varied spatially and temporally. Massive framework species, including Siderastrea siderea, Colpophyllia natans, and Montastraea annularis, along with relatively smaller colonies of Meandrina meandrites and Dichocoenia stokesi, were most susceptible to disease. Temperature, sedimentation, and dissolved inorganic nitrogen were positively correlated with BBD infections. Furthermore, experimental nutrient enrichment exacerbated coral tissue loss to BBD both in situ and in vivo. Profiling of BBD microbial communities using length heterogeneity PCR revealed variation over space and time, with significantly distinct bacterial assemblages in the NFK, LSI, and US Virgin Islands. ^ This study contributes to knowledge of the relationship between coral diseases and the environment, and facilitates predictions regarding potential changes in coral reef communities under differing environmental conditions. Additionally, this research provides further understanding of coral disease dynamics at both the host and microbial pathogen levels.^

An assessment of the microbial contribution to aquatic dissolved organic nitrogen using amino acid enantiomeric ratios

There is increasing evidence that certain microbially-derived compounds may account for part of the aquatic dissolved organic nitrogen (DON) pool. Enantiomeric ratios of amino acids were used to assess the microbial input to the DON pool in the Florida Everglades, USA. Elevated levels of d-alanine, d-aspartic acid, d-glutamic acid and d-serine indicated the presence of peptidoglycan in the samples. The estimated peptidoglycan contribution to amino acid nitrogen ranged from 2.8 ± 0.1% to 6.4 ± 0.9%, increasing with salinity from freshwater to coastal waters. The distribution of individual d-amino acids in the samples suggests additional inputs to DON, possibly from archaea or from abiotic racemization of l-amino acids.

Chemical characterization of dissolved organic nitrogen in an oligotrophic subtropical coastal ecosystem

Water samples were collected from rivers and estuarine environments within the Florida Coastal Everglades (FCE) ecosystem, USA, and ultrafiltered dissolved organic matter (UDOM; 1 kDa) was isolated for characterization of its source, bioavailability and diagenetic state. A combination of techniques, including 15N cross-polarization magic angle spinning nuclear magnetic resonance (15N CPMAS NMR) and X-ray photoelectron spectroscopy (XPS), were used to analyze the N components of UDOM. The concentrations and compositions of total hydrolysable amino acids (HAAs) were analyzed to estimate UDOM bioavailability and diagenetic state. Optical properties (UV–visible and fluorescence) and the stable isotope ratios of C and N were measured to assess the source and dynamics of UDOM. Spectroscopic analyses consistently showed that the major N species of UDOM are in amide form, but significant contributions of aromatic-N were also observed. XPS showed a very high pyridinic-N concentration in the FCE–UDOM (21.7 ± 2.7%) compared with those in other environments. The sources of this aromatic-N are unclear, but could include soot and charred materials from wild fires. Relatively high total HAA concentrations (4 ± 2% UDOC or 27 ± 4% UDON) are indicative of bioavailable components, and HAA compositions suggest FCE–UDOM has not undergone extensive diagenetic processing. These observations can be attributed to the low microbial activity and a continuous supply of fresh UDOM in this oligotrophic ecosystem. Marsh plants appear to be the dominant source of UDOM in freshwater regions of the FCE, whereas seagrasses and algae are the dominant sources of UDOM in Florida Bay. This study demonstrates the utility of a multi-technique and multi-proxy approach to advance our understanding of DON biogeochemistry.

Dissolved Black Nitrogen (DBN) in Freshwater Environments

Biomass burning results in the formation and accumulation of pyrogenic products such as black carbon (BC) and black nitrogen (BN) in soils. The ubiquitous presence of pyrogenic products in natural dissolved organic matter (DOM) and potential implications in global carbon cycling have recently been reported. However, little is known about the environmental dynamics or the importance in the global N cycle of dissolved BN (DBN; or heteroaromatic N). Here we report the coupling between DBN and dissolved BC (DBC) in ultrafiltered DOM from six headwater streams across a climatic region of North America, suggesting similar combustion sources, and that DOC may play an important role in the translocation of soil BN to the dissolved phase. The export of potentially recalcitrant riverine DBN to the ocean may affect the biogeochemical cycling of N and possibly the microbial community structure in aquatic environments.