The role of dissolved organic matter bioavailability in promoting phytoplankton blooms in Florida Bay

The clear, shallow, oligotrophic waters of Florida Bay are characterized by low phytoplankton biomass, yet periodic cyanobacteria and diatom blooms do occur. We hypothesized that allochthonous dissolved organic matter (DOM) was providing a subsidy to the system in the form of bound nutrients. Water from four bay sites was incubated under natural light and dark conditions with enrichments of either DOM ( > 1 kD, 2×DOM) or inorganic nutrients (N+P). Samples were analyzed for bacterial numbers, bacterial production, phytoplankton biomass, phytoplankton community structure, and production, nutrients, and alkaline phosphatase (AP) activity. The influence of 2×DOM enrichment on phytoplankton biomass developed slowly during the incubations and was relatively small compared to nutrient additions. Inorganic nutrient additions resulted in an ephemeral bloom characterized initially as cyanobacterial and brown algae but which changed to dinoflagellate and/or brown algae by day six. The DIN:TP ratio decreased 10-fold in the N+P treatments as the system progressed towards N limitation. This ratio did not change significantly for 2×DOM treatments. In addition, these experiments indicated that both autotrophic and heterotrophic microbial populations in Florida Bay may fluctuate in their limitation by organic and inorganic nutrient availability. Both N+P and 2×DOM enrichments revealed significant and positive response in bioavailability of dissolved organic carbon (BDOC). Potential BDOC ranged from 1.1 to 35.5%, with the most labile forms occurring in Whipray Basin. BDOC at all sites was stimulated by the 2×DOM addition. Except for Duck Key, BDOC at all sites was also stimulated by the addition of N+P. BDOC was lower in the dry season than in the wet season (5.56% vs. 16.86%). This may be explained by the distinct chemical characteristics of the DOM produced at different times of year. Thus, both the heterotrophic and autotrophic microbial communities in Florida Bay are modulated by bioavailability of DOM. This has ramifications for the fate of DOM from the Everglades inputs, implicating DOM bioavailability as a contributing factor in regulating the onset, persistence, and composition of phytoplankton blooms.

Phytoplankton bloom status: Chlorophyll a biomass as an indicator of water quality condition in the southern estuaries of Florida, USA

Altered freshwater inflows have affected circulation, salinity, and water quality patterns of Florida Bay, in turn altering the structure and function of this estuary. Changes in water quality and salinity and associated loss of dense turtle grass and other submerged aquatic vegetation (SAV) in Florida Bay have created a condition in the bay where sediments and nutrients have been regularly disturbed, frequently causing large and dense phytoplankton blooms. These algal and cyanobacterial blooms in turn often cause further loss of more recently established SAV, exacerbating the conditions causing the blooms. Chlorophyll a (CHLA) was selected as an indicator of water quality because it is an indicator of phytoplankton biomass, with concentrations reflecting the integrated effect of many of the water quality factors that may be altered by restoration activities. Overall, we assessed the CHLA indicator as being (1) relevant and reflecting the state of the Florida Bay ecosystem, (2) sensitive to ecosystem drivers (stressors, especially nutrient loading), (3) feasible to monitor, and (4) scientifically defensible. Distinct zones within the bay were defined according to statistical and consensual information. Threshold levels of CHLA for each zone were defined using historical data and scientific consensus. A presentation template of condition of the bay using these thresholds is shown as an example of an outreach product.

Shifting N and P limitation along a north-south gradient of mangrove estuaries in South Florida

A multivariate statistical analysis was applied to a 10 year, multiparameter data set in an effort to describe the spatial dependence and inherent variation of water quality patterns in the mangrove estuaries of Ten Thousand Islands – Whitewater Bay area. Principal component analysis (PCA) of 16 water quality parameters collected monthly resulted in five groupings, which explained 72.5% of the variance of the original variables. The “Organic” component (PCI) was composed of alkaline phosphatase activity, total organic nitrogen, and total organic carbon; the “Dissolved Inorganic N” component (PCII) contained NO 3 − , NO 2 − , and NH 4 + ; the “Phytoplankton” component (PCIII) was made up of total phosphorus, chlorophyll a, and turbidity; dissolved oxygen and temperature were inversely related (PCIV); and salinity and soluble reactive phosphorus made up PCV. A cluster analysis of the mean and SD of PC scores resulted in the spatial aggregation of the 47 fixed stations into six classes having similar water quality, which we defined as: Mangrove Rivers, Whitewater Bay, Gulf Islands, Coot Bay, Blackwater River, and Inland Waterway. Marked differences in physical, chemical, and biological characteristics among classes were illustrated by this technique. Comparison of medians and variability of parameters among classes allowed large scale generalizations as to underlying differences in water quality in these regions. A strong south to north gradient in estuaries from high N - low P to low N - high P was ascribed to marked differences in landuse, freshwater input, geomorphology, and sedimentary geology along this tract. The ecological significance of this gradient discussed along with potential effects of future restoration plans.

Mangrove production and carbon sinks: A revision of global budget estimates

Mangrove forests are highly productive but globally threatened coastal ecosystems, whose role in the carbon budget of the coastal zone has long been debated. Here we provide a comprehensive synthesis of the available data on carbon fluxes in mangrove ecosystems. A reassessment of global mangrove primary production from the literature results in a conservative estimate of ∼218 ± 72 Tg C a−1. When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at ∼112 ± 85 Tg C a−1, equivalent in magnitude to ∼30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.

Pink shrimp as an indicator for restoration of everglades ecosystems

The pink shrimp, Farfantepenaeus duorarum, familiar to most Floridians as either food or bait shrimp, is ubiquitous in South Florida coastal and offshore waters and is proposed as an indicator for assessing restoration of South Florida's southern estuaries: Florida Bay, Biscayne Bay, and the mangrove estuaries of the lower southwest coast. Relationships between pink shrimp and salinity have been determined in both field and laboratory studies. Salinity is directly relevant to restoration because the salinity regimes of South Florida estuaries, critical nursery habitat for the pink shrimp, will be altered by changes in the quantity, timing, and distribution of freshwater inflow planned as part of the Comprehensive Everglades Restoration Project (CERP). Here we suggest performance measures based on pink shrimp density (number per square meter) in the estuaries and propose a restoration assessment and scoring scheme using these performance measures that can readily be communicated to managers, policy makers, and the interested public. The pink shrimp is an appropriate restoration indicator because of its ecological as well as its economic importance and also because scientific interest in pink shrimp in South Florida has produced a wealth of information about the species and relatively long time series of data on both juveniles in estuarine nursery habitats and adults on the fishing grounds. We suggest research needs for improving the pink shrimp performance measure.

Negative relationships between the nutrient and carbohydrate content of the seagrass Thalassia testudinum

This study documents relationships between plant nutrient content and rhizome carbohydrate content of a widely distributed seagrass species, Thalassia testudinum, in Florida. Five distinct seagrass beds were sampled for leaf nitrogen, leaf phosphorus, and rhizome carbohydrate content from 1997 to 1999. All variables displayed marked intra- and inter- regional variation. Elemental ratios (mean N:P ± S.E.) were lowest for Charlotte Harbor (9.9 ± 0.2) and highest for Florida Bay (53.5 ± 0.9), indicating regional shifts in the nutrient content of plant material. Rhizome carbohydrate content (mean ± S.E.) was lowest for Anclote Keys (21.8 ± 1.6 mg g−1 FM), and highest for Homosassa Bay (40.7 ± 1.7 mg g−1 FM). Within each region, significant negative correlations between plant nutrient and rhizome carbohydrate content were detected; thus, nutrient-replete plants displayed low carbohydrate content, while nutrient-deplete plants displayed high carbohydrate content. Spearman's rank correlations between nutrient and carbohydrate content varied from a minimum in Tampa Bay (ρ = −0.2) to a maximum in Charlotte Harbor (ρ = −0.73). Linear regressions on log-transformed data revealed similar trends. This consistent trend across five distinct regions suggests that nutrient supply may play an important role in the regulation of carbon storage within seagrasses. Here we present a new hypothesis for studies which aim to explain the carbohydrate dynamics of benthic plants.

Interspecific variation in the elemental and stable isotope content of seagrasses in South Florida

The elemental (C, N, and P) and isotope (δ13C, δ15N) content of leaves of the seagrasses Thalassia testudinum, Halodule wrightii, and Syringodium filiforme were measured across a 10 000 km2 survey of the seagrass communities of South Florida, USA, in 1999 and 2000. Trends at local and broad spatial scales were compared to examine interspecific variation in the seagrass characteristics often used as ecological indicators. The elemental and stable isotope contents of all species were variable and demonstrated marked interspecific variation. At broad spatial scales, mean N:P ratios were lowest for T. testudinum (36.5 ± 1.1) and S. filiforme (38.9 ± 1.3), and highest for H. wrightii (44.1 ± 1.8). Stable carbon isotope ratios (δ13C) were highest for S. filiforme (–6.2 ± 0.2‰), intermediate for T. testudinum (–8.6 ± 0.2‰), and lowest for H. wrightii (–10.6 ± 0.3‰). Stable nitrogen isotopes (δ15N) were heaviest for T. testudinum (2.0 ± 0.1‰), and lightest for H. wrightii (1.0 ± 0.3‰) and S. filiforme (1.6 ± 0.2‰). Site depth was negatively correlated to δ13C for all species, while δ15N was positively correlated to depth for H. wrightii and S. filiforme. Similar trends were observed in local comparisons, suggesting that taxon-specific physiological/ecological properties strongly control interspecific variation in elemental and stable isotope content. Temporal trends in δ13C were measured, and revealed that interspecific variation was displayed throughout the year. This work documents interspecific variation in the nutrient dynamics of 3 common seagrasses in South Florida, indicating that interpretation of elemental and stable isotope values needs to be species specific.

Novel methodology for in situ carbon dioxide enrichment of benthic ecosystems

Future climate change will likely represent a major stress to shallow aquatic and coastal marine communities around the world. Most climate change research, particularly in regards to increased pCO2 and ocean acidification, relies on ex situ mesocosm experimentation, isolating target organisms from their environment. Such mesocosms allow for greater experimental control of some variables, but can often cause unrealistic changes in a variety of environmental factors, leading to “bottle effects.” Here we present an in situ technique of altering dissolved pCO2within nearshore benthic communities (e.g., macrophytes, algae, and/or corals) using submerged clear, open-top chambers. Our technique utilizes a flow-through design that replicates natural water flow conditions and minimizes caging effects. The clear, open-top design additionally ensures that adequate light reaches the benthic community. Our results show that CO2 concentrations and pH can be successfully manipulated for long durations within the open-top chambers, continuously replicating forecasts for the year 2100. Enriched chambers displayed an average 0.46 unit reduction in pH as compared with ambient chambers over a 6-month period. Additionally, CO2 and HCO3 – concentrations were all significantly higher within the enriched chambers. We discuss the advantages and disadvantages of this technique in comparison to other ex situ mesocosm designs used for climate change research.

Periphyton responses to eutrophication in the Florida Everglades: Cross-system patterns of structural and compositional change

We examined periphyton along transects in five Everglades marshes and related compositional and functional aspects to phosphorus(P ) gradients caused by enriched inflows. Results were compared to those of a P-addition experiment in a pristine Everglades marsh. While the water total P (TP) concentration was not related to P load in the marshes or experiment the concentration of TP in periphyton was strongly correlated with the distance from the P source. Increased P concentration in periphyton was associated with a loss of biomass,p articularly of the calcifying mat-forming matrix, regardless of the growth form of the periphyton (epiphytic, floating,or epilithic). Diatom species composition was also strongly related to P availability, but the TP optima of many species varied among marshes. Enriched periphyton communities were found 14 km downstream of P inputs to one marsh that has been receiving enhanced P loads for decades, where other studies using different biotic indicators show negligible change in the same marsh. Although recovery trajectories are unknown, periphyton indicators should serve as excellent metrics for the progression or amelioration of P-related effects in the Everglades.

Multidecadal climate oscillations detected in a transparency record from a subtropical Florida lake

Synchronous interannual variability in water transparency observed in neighboring lakes has been linked to regional precipitation and resultant runoff of dissolved organic material, but many climate forcings oscillate over time scales longer than most limnological records can detect. A strong relationship (R2 5 0.86) between transparency and the previous two years’ rainfall and lake stage in a 25-yr record from a Florida lake enabled us to hindcast transparency from a longer 75-yr record of rainfall and lake stage. Predictions revealed a ,30-yr cycle in transparency linked to the Atlantic Multidecadal Oscillation (AMO). Transparency was greatest (4–8 m) in the cool phase of the AMO (,1962–1993) associated with below-average rainfall in south Florida and lowest (0.1– 3.0 m) during two warm phases (,1932–1961, 1994–present) associated with above-average, but more variable, annual rainfall. Models that predict effects of large-scale hydrologic restoration projects on solute export from South Florida’s expansive wetlands need to account for recent entry into a warm AMO phase, where teleconnections between the AMO phases and runoff are opposite of those shown for the U.S. interior.

Spatial scale and abundance patterns of large fish communities in freshwater marshes of the Florida Everglades

Anthropogenic habitat alterations and water-management practices have imposed an artificial spatial scale onto the once contiguous freshwater marshes of the Florida Everglades. To gain insight into how these changes may affect biotic communities, we examined whether variation in the abundance and community structure of large fishes (SL . 8 cm) in Everglades marshes varied more at regional or intraregional scales, and whether this variation was related to hydroperiod, water depth, floating mat volume, and vegetation density. From October 1997 to October 2002, we used an airboat electrofisher to sample large fishes at sites within three regions of the Everglades. Each of these regions is subject to unique watermanagement schedules. Dry-down events (water depth , 10 cm) occurred at several sites during spring in 1999, 2000, 2001, and 2002. The 2001 dry-down event was the most severe and widespread. Abundance of several fishes decreased significantly through time, and the number of days post-dry-down covaried significantly with abundance for several species. Processes operating at the regional scale appear to play important roles in regulating large fishes. The most pronounced patterns in abundance and community structure occurred at the regional scale, and the effect size for region was greater than the effect size for sites nested within region for abundance of all species combined, all predators combined, and each of the seven most abundant species. Non-metric multi-dimensional scaling revealed distinct groupings of sites corresponding to the three regions. We also found significant variation in community structure through time that correlated with the number of days post-dry-down. Our results suggest that hydroperiod and water management at the regional scale influence large fish communities of Everglades marshes.

Comparative study of dissolved organic matter from groundwater and surface water in the Florida coastal Everglades using multi-dimensional spectrofluorometry combined with multivariate statistics

Dissolved organic matter (DOM) in groundwater and surface water samples from the Florida coastal Everglades were studied using excitation–emission matrix fluorescence modeled through parallel factor analysis (EEM-PARAFAC). DOM in both surface and groundwater from the eastern Everglades S332 basin reflected a terrestrial-derived fingerprint through dominantly higher abundances of humic-like PARAFAC components. In contrast, surface water DOM from northeastern Florida Bay featured a microbial-derived DOM signature based on the higher abundance of microbial humic-like and protein-like components consistent with its marine source. Surprisingly, groundwater DOM from northeastern Florida Bay reflected a terrestrial-derived source except for samples from central Florida Bay well, which mirrored a combination of terrestrial and marine end-member origin. Furthermore, surface water and groundwater displayed effects of different degradation pathways such as photodegradation and biodegradation as exemplified by two PARAFAC components seemingly indicative of such degradation processes. Finally, Principal Component Analysis of the EEM-PARAFAC data was able to distinguish and classify most of the samples according to DOM origins and degradation processes experienced, except for a small overlap of S332 surface water and groundwater, implying rather active surface-to-ground water interaction in some sites particularly during the rainy season. This study highlights that EEM-PARAFAC could be used successfully to trace and differentiate DOM from diverse sources across both horizontal and vertical flow profiles, and as such could be a convenient and useful tool for the better understanding of hydrological interactions and carbon biogeochemical cycling.

Sediment and Nutrient Deposition Associated with Hurricane Wilma in Mangroves of the Florida Coastal Everglades

The distribution of mangrove biomass and forest structure along Shark River estuary in the Florida Coastal Everglades (FCE) has been correlated with elevated total phosphorus concentration in soils thought to be associated with storm events. The passage of Hurricane Wilma across Shark River estuary in 2005 allowed us to quantify sediment deposition and nutrient inputs in FCE mangrove forests associated with this storm event and to evaluate whether these pulsing events are sufficient to regulate nutrient biogeochemistry in mangrove forests of south Florida. We sampled the spatial pattern of sediment deposits and their chemical properties in mangrove forests along FCE sites in December 2005 and October 2006. The thickness (0.5 to 4.5 cm) of hurricane sediment deposits decreased with distance inland at each site. Bulk density, organic matter content, total nitrogen (N) and phosphorus (P) concentrations, and inorganic and organic P pools of hurricane sediment deposits differed from surface (0–10 cm) mangrove soils at each site. Vertical accretion resulting from this hurricane event was eight to 17 times greater than the annual accretion rate (0.30± 0.03 cm year−1) averaged over the last 50 years. Total P inputs from storm-derived sediments were equivalent to twice the average surface soil nutrient P density (0.19 mg cm−3). In contrast, total N inputs contributed 0.8 times the average soil nutrient N density (2.8 mg cm−3). Allochthonous mineral inputs from Hurricane Wilma represent a significant source of sediment to soil vertical accretion rates and nutrient resources in mangroves of southwestern Everglades. The gradient in total P deposition to mangrove soils from west to east direction across the FCE associated with this storm event is particularly significant to forest development due to the P-limited condition of this carbonate ecosystem. This source of P may be an important adaptation of mangrove forests in the Caribbean region to projected impacts of sea-level rise.

Patterns of Root Dynamics in Mangrove Forests Along Environmental Gradients in the Florida Coastal Everglades, USA

Patterns of mangrove vegetation in two distinct basins of Florida Coastal Everglades (FCE), Shark River estuary and Taylor River Slough, represent unique opportunities to test hypotheses that root dynamics respond to gradients of resources, regulators, and hydroperiod. We propose that soil total phosphorus (P) gradients in these two coastal basins of FCE cause specific patterns in belowground biomass allocation and net primary productivity that facilitate nutrient acquisition, but also minimize stress from regulators and hydroperiod in flooded soil conditions. Shark River basin has higher P and tidal hydrology with riverine mangroves, in contrast to scrub mangroves of Taylor basin with more permanent flooding and lower P across the coastal landscape. Belowground biomass (0–90 cm) of mangrove sites in Shark River and Taylor River basins ranged from 2317 to 4673 g m-2, with the highest contribution (62–85%) of roots in the shallow root zone (0–45 cm) compared to the deeper root zone (45–90 cm). Total root productivity did not vary significantly among sites and ranged from 407 to 643 g m-2 y-1. Root production in the shallow root zone accounted for 57–78% of total production. Root turnover rates ranged from 0.04 to 0.60 y-1 and consistently decreased as the root size class distribution increased from fine to coarse roots, indicating differences in root longevity. Fine root biomass was negatively correlated with soil P density and frequency of inundation, whereas fine root turnover decreased with increasing soil N:P ratios. Lower P availability in Taylor River basin relative to Shark River basin, along with higher regulator and hydroperiod stress, confirms our hypothesis that interactions of stress from resource limitation and long duration of hydroperiod account for higher fine root biomass along with lower fine root production and turnover. Because fine root production and organic matter accumulation are the primary processes controlling soil formation and accretion in scrub mangrove forests, root dynamics in the P-limited carbonate ecosystem of south Florida have a major controlling role as to how mangroves respond to future impacts of sealevel rise.

Developing performance measures of mangrove wetlands using simulation models of hydrology, nutrient biogeochemistry, and community dynamics

The goal of mangrove restoration projects should be to improve community structure and ecosystem function of degraded coastal landscapes. This requires the ability to forecast how mangrove structure and function will respond to prescribed changes in site conditions including hydrology, topography, and geophysical energies. There are global, regional, and local factors that can explain gradients of regulators (e.g., salinity, sulfides), resources (nutrients, light, water), and hydroperiod (frequency, duration of flooding) that collectively account for stressors that result in diverse patterns of mangrove properties across a variety of environmental settings. Simulation models of hydrology, nutrient biogeochemistry, and vegetation dynamics have been developed to forecast patterns in mangroves in the Florida Coastal Everglades. These models provide insight to mangrove response to specific restoration alternatives, testing causal mechanisms of system degradation. We propose that these models can also assist in selecting performance measures for monitoring programs that evaluate project effectiveness. This selection process in turn improves model development and calibration for forecasting mangrove response to restoration alternatives. Hydrologic performance measures include soil regulators, particularly soil salinity, surface topography of mangrove landscape, and hydroperiod, including both the frequency and duration of flooding. Estuarine performance measures should include salinity of the bay, tidal amplitude, and conditions of fresh water discharge (included in the salinity value). The most important performance measures from the mangrove biogeochemistry model should include soil resources (bulk density, total nitrogen, and phosphorus) and soil accretion. Mangrove ecology performance measures should include forest dimension analysis (transects and/or plots), sapling recruitment, leaf area index, and faunal relationships. Estuarine ecology performance measures should include the habitat function of mangroves, which can be evaluated with growth rate of key species, habitat suitability analysis, isotope abundance of indicator species, and bird census. The list of performance measures can be modified according to the model output that is used to define the scientific goals during the restoration planning process that reflect specific goals of the project.