Department of Biological Sciences
Florida International University
Miami, FL 33199
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Abstract
There is a consensus that many changes in coastal marine ecosystems worldwide are anthropogenically driven. It has been asserted that anthropogenic impacts are responsible for degradation of coastal marine ecosystems in the Florida Keys, however there is a paucity of data that support this conclusion. Accurate quantitative assessments of the effects of human activity on the coastal marine ecosystems of the Florida Keys were needed so that coastal residents, scientists, and resource managers may focus their conservation, research, and legislative efforts in the appropriate direction. This project entails an investigation of nearshore (<1 km from shore) benthic communities of the Florida Keys. It was designed to identify spatial and temporal variations within nearshore benthic communities and their associated nutrient regimes and to determine if these variations may be associated with human land use activity in the Florida Keys. Working hypotheses included:
H1: Nearshore benthic communities and their associated nutrient regimes exhibit spatial/temporal variation throughout the Florida Keys.
H2: There is a significant relationship between human land use activity and spatial/temporal variation of nearshore benthic communities and their associated nutrient regimes throughout the Florida Keys.
The project began with the creation of maps of the current distribution and composition of nearshore benthic communities using intensive surveys and recent aerial photographs. Next, historic aerial photographs were used to construct a complete time series of maps at multiple sites within the study area. The nature of changes within nearshore benthic communities at those sites was investigated. Nutrient samples were also collected near the time series sites to characterize the nutrient regimes of nearshore benthic communities. Finally, all project data and available Florida Keys land use activity data were incorporated into a geographic information system (GIS) database. Database queries and spatial analyses were conducted to explore relationships between nearshore benthic communities, nearshore nutrient regimes, and land use activities in the Florida Keys. The results of this project may be incorporated into the development of a model that predicts the effects of land use activity and anthropogenic nutrient enrichment on nearshore benthic communities of the Florida Keys.
Introduction
The coastal marine ecosystems of the Florida Keys (Figure 1) comprise three dominant habitats: mangroves, seagrasses, and coral reefs. Mangroves are a dominant feature of the land-sea margin throughout the Florida Keys. Seagrasses are the principle component of benthic marine habitats in the Florida Keys; they occupy over 70% of the FKNMS. The structure and distribution of seagrass communities varies throughout the Florida Keys; they form extensive meadows and patchy beds, and are also found scattered throughout other habitats. Coral reefs, patch reefs, and hardbottom habitats cover approximately 7%, 1% and 19% of the FKNMS, respectively (Figure 2).
The unifying geologic feature of the Florida Keys is Key Largo Limestone, a Pleistocene formation of lithified fossil coral, which extends as a solid mass from Miami to the Dry Tortugas. The keys may be divided geologically into two main sections: 1) the upper and middle keys, from Soldier Key to Newfound Harbor Keys, where the Key Largo Limestone is exposed and 2) the lower keys, Big Pine Key and beyond, where the Key Largo Limestone is overlain with another Pleistocene formation, Miami Oolite. Additional facies, including Holocene reefs, lime muds, and modern carbonate sands are found throughout the Florida Keys. The distribution of these geologic formations contributes to variations in the distribution of marine habitats in the Florida Keys (Figure 3).
The hydrologic nature of the Florida Keys is also influenced by the distribution of these geologic formations. The permeability and porosity of limestone facilitates seepage of groundwater. It appears that anthropogenic nutrients could not only enter nearshore surface waters via runoff or canals, but may plausibly be transported laterally to offshore subsurface waters through the carbonate substrate . The presence of submarine seeps has indicated that groundwater may be transported to nearshore habitats. Anecdotal evidence suggests that seeps may occur offshore where areas of exposed limestone come into contact with overlying waters. Thus, there is potential for natural or anthropogenic nutrient enrichment to reach both nearshore and offshore communities.
It has been determined that in many places in the Florida Keys, biomass and productivity of benthic plants are nutrient limited and initially respond positively to nutrient enrichment. There is also evidence that suggests nutrient limitation, and consequently plant response to nutrient enrichment, may differ throughout the Florida Keys. Studies have shown that there is more phosphorous available offshore, near the reef tract, while groundwater and Florida Bay water are relatively rich in available nitrogen (Figure 4). It can be expected that the effects of nutrient enrichment may vary due to the different nutrient limitations and regimes of nearshore and offshore benthic communities.
A great deal of contemporary research is focused on the effects of nutrient enrichment on coastal marine ecosystems. Many studies have implicated anthropogenic eutrophication as the major cause of decline in coastal marine ecosystems, however, despite many comprehensive investigations, it has proven difficult to define linkages between land use activity and changes in coastal marine ecosystems. A diverse array of investigations has been conducted in the Florida Keys (Table 1), yet few investigations have focused specifically on the relationships between land use activity and coastal marine ecosystems. Brian E. Lapointe and his colleagues have conducted some preliminary research focusing on the effects of anthropogenic nutrient inputs on coastal marine ecosystems in the Florida Keys. Studies by Lapointe et al. indicate that anthropogenic nutrient enrichment of groundwater may be occurring locally in the Florida Keys and that nutrient enrichment can have an effect on benthic marine communities. However, the implied hypothesis, that anthropogenic nutrient enrichment affects coastal marine ecosystems of the Florida Keys, has never been subjected to rigorous testing by any investigator.
Methods
Approximately 1500 sampling sites were located in the nearshore zone (<1 km from the shoreline) in four areas throughout the Florida Keys: Key Largo, Marathon, Big Pine, and Key West (Figure 6). These four areas were selected because they include expanses of heavily developed urban areas adjacent to sparsely developed coastal ecosystems. The distribution of study sites within these four areas provided excellent representations of nearshore benthic communities and nutrient regimes throughout the Florida Keys.
Intensive surveys were used to document the distribution, and composition of current nearshore benthic communities. Community composition was assessed using the Braun Blanquet quadrat method used by the EPA Water Quality Protection Plan seagrass monitoring project in the Florida Keys National Marine Sanctuary (FKNMS). The percent coverage of benthic taxa (Table 3) was recorded from ten 0.25 m2 quadrats haphazardly located within 10 m of the DGPS coordinates. Community composition data was collected at approximately 1500 sites, with an estimated sampling density of 6 sites per km2 (Figures 7a-d). Community composition data was then entered into a Microsoft Access database and exported into ArcView and SAS for further spatial and statistical analyses. Nearshore benthic community classes were identified by a consensus of classification approaches. These classes were used to assist in developing maps of current nearshore benthic communities throughout the study area.
An assessment of constancy and change in nearshore benthic communities of the Florida Keys was conducted at multiple sites within the study area (Figures 9a-d). At each site, a complete set of Florida Department of Transportation (FDOT) black and white aerial photographs was used to construct a time series of benthic community maps. Temporal changes in the distribution of benthic macrophytes were recorded at each site using the time series maps.
The nutrient regimes associated with nearshore benthic communities were characterized by sampling of seagrasses, epiphytes, and sediments. These samples were collected at permanently located transects near each of the time series sites described above (Figures 11a-d). These data were used to determine spatial variability in nearshore nutrient regimes as well as the nature of nearshore to offshore nutrient gradients.
A Geographic Information System (GIS) was used to assist in investigation of possible relationships between nearshore benthic communities and land use activities of the Florida Keys. Spatial analyses were conducted in ArcView to explore: 1) relationships between land use and the current distribution of nearshore benthic communities, 2) relationships between land use and changes in nearshore benthic communities, and 3) relationships between land use and nearshore nutrient regimes. Additional statistical analyses of relationships between land use and the distribution of nearshore benthic communities and nearshore nutrient regimes were conducted using SAS.
Results
The consensus classification of community composition data collected at nearly 1400 sites throughout the Florida Keys study area includes eight classes. Four of the eight classes are dominated by seagrasses; one class is a community of mixed macrophytes, and the remaining three classes are hardbottom communities (Table 9). Out of the 1367 sites included in the classification analyses, 1329 sites fell into one of the eight classes; 38 sites were not assigned to a class (Figure 14). A little over half of the sites were classified as seagrass community, while approximately one third were classified as hardbottom community. The percentage of sites belonging to each class varied among study areas (Table 10), with a significant decrease in seagrass communities and increase in hardbottom communities moving southwest from Key Largo out to Key West (Figure 15). The distribution of the nearshore benthic community classes throughout the four study areas is shown in Figures 16 a-d. All nearshore benthic community data was subjected to modeling and statistical analyses in SAS; those results are included below.
Time series analyses of the black and white DOT aerial photographs reveals very little change in the distribution of nearshore benthic communities in the Florida Keys since 1959 (Figure 17). There are no significant differences in the amount of keys-wide benthic macrophyte cover with respect to time (1959-1997, six time steps), location (oceanside or bayside), or land use (heavily or slightly developed). However, there are clear differences in the magnitude and direction of the minimal changes detected with respect to study area. The mean temporal change at most Key Largo and Marathon sites were positive, reflecting small net increases, while the mean temporal change at most Big Pine and Key West sites were negative, reflecting slight net decreases. The mean temporal variability was significantly higher in Key Largo and Marathon, indicating a greater number of points at sites in those study areas was changing through time. The mean temporal variability was significantly lower in Big Pine and Key West, indicating more stability at sites in those study areas (Figures 18a-b). No time series data was subjected to modeling or analyses in SAS, since there were no significant relationships between temporal data and the location/ land use classifications assigned to each transect.
Preliminary analyses of Thalassia testudinum, sediment, and epiphyte samples collected at 32 transects did not reveal any significant keys-wide trends in nutrient parameters with respect to location (oceanside or bayside), distance from shore (50 m, 100 m, 250 m, or 500 m), or land use (heavily or slightly developed). However, ArcView plots of nutrient data indicated that potentially significant relationships might exist within study areas (Figures 19a-d). All nutrient data were therefore included for modeling and statistical analyses in SAS; those results are included below.
All nearshore benthic community data and nearshore nutrient regime data were subjected to modeling and statistical analyses in SAS. Two preliminary modeling approaches included a general linear model (Figure 20) and a radial model (Figure 21). Both models revealed a number of significant relationships between nearshore benthic community data, nearshore nutrient data, and FLUCCS land use data within study areas. The nature of the significant relationships varied between study areas; individual relationships were rarely significant across all study areas. In addition, significant relationships at a high level of taxonomic resolution (individual taxa) were often obscured when examined at a lower level of taxonomic resolution (taxa group). The models also uncovered strong significant relationships between substrate (sediment type and depth), land use, nearshore benthic communities, and their associated nutrient regimes throughout the Florida Keys. The results of both modeling approaches are summarized in appendices 1a (taxa groups), 1b (individual taxa), 1c (nutrients), 2a (taxa groups), 2b (individual taxa), and 2c (nutrients).
Conclusion
This project was designed to identify spatial and temporal variations within nearshore benthic communities and their associated nutrient regimes and to determine if these variations may be associated with human land use activity in the Florida Keys. The first working hypothesis, that nearshore benthic communities and their associated nutrient regimes exhibit spatial or temporal variation throughout the Florida Keys, has been conclusively addressed. We have determined that both nearshore benthic communities and their associated nutrient regimes do exhibit spatial variation throughout the Florida Keys. However, nearshore benthic communities were found to exhibit very little temporal variation through the past 40 years, even in the face of tremendous land development in the Florida Keys. Naturally, any changes that occurred within the study areas prior to 1960 (or outside of the study areas altogether) remain undocumented.
The second working hypothesis, that there is a significant relationship between human land use activity and spatial or temporal variation of nearshore benthic communities and their associated nutrient regimes throughout the Florida Keys, merits further investigation. The preliminary models identified potential relationships between nearshore data and land use data within study areas, but these relationships rarely held true throughout the Florida Keys. In addition, the results indicate that substrate, not land use, may be the most important factor associated benthic community composition; a fact which is potentially confounding since human land use and substrate are significantly correlated. It must be noted, that due to the coarse nature of the FLUCCS land use data used in the modeling approaches, the analyses have little power to detect impacts at spatial scales on the order of 10s of meters (small impacts, such as canals or outfalls), nor can they detect impacts that range over spatial scales greater than a few kilometers (including any land use impact that is strong or diffuse enough to affect all of the Florida Keys).
We recommend that this project's efforts should be extended into a continuing modeling effort. Suggested improvements include 1) a geographically expanded nearshore benthic community data set- with composition data collected at the highest possible level of taxonomic resolution and substrate information included with each sample; 2) a more comprehensive nearshore nutrient sampling program- sporadic water quality sampling in residential canals does not provide researchers with an ecologically relevant foundation on which to address questions of possible nearshore eutrophication in the Florida Keys; 3) a more appropriate land use data set- a spatially accurate, quantitative land use data set would enable researchers to fully realize the potential of the enormous nearshore benthic community database generated by this project.