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SiO2 and turbidity, and highest inorganic nitrogen concentrations. Finally, South
Biscayne Bay, surrounded by wetlands and practically devoid of urban development in
its watershed, has the lowest salinity and highest total nitrogen (TN), total organic
nitrogen (TON) and total organic carbon (TOC).
We calculated descriptive statistics and performed long-term trend exploration of
the time-series, to gain insight into patterns of behavior along the period of record
(POR) for all relevant biogeochemical parameters. This exploration was performed with
Akritas-Theil Sen and Z-score cumulative sum charts (Z-CUSUM) methodologies. The
spatial-temporal variability within estuaries and coastal waters in individual South
Florida basins called for a holistic approach to basin segmentation to account for
variability not only dictated by a given nutrient concentration level, but by the
combination of stressors conditions (nutrients, water clarity, climate, weather, extreme
events, geomorphology, circulation and exchange, management, etc.). Basin
segmentation was accomplished with an objective classification of station combining
Principal Component Analysis (PCA) and Hierarchical Clustering methods in tandem.
Besides statistics, spatial extension and pattern, geomorphology, water circulation and
benthic ecosystem distribution played a major role on the final delineation of 40
segments extending from Biscayne Bay in the east to Dry Tortugas in the southwest
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and to Pine Island Sound in the northwest. This segmentation, or a modification of it,
has been adopted by both, EPA and FDEP as fundamental framework for the derivation
of numeric nutrient criteria for South Florida estuaries and coastal waters.
Substantial changes and climate-driven regime shifts have been documented for
water quality in South Florida. The most relevant changes we identified in Biscayne Bay
were those occurring in the early-to-mid nineties from lower to higher precipitation rates,
leading in turn to a generalized region-wide decline in water nutrients and chlorophyll a
(CHL-a) concentrations (Briceño and Boyer 2010). Based upon a detailed literature
review and a recent assessment of natural resource conditions in Biscayne Bay and
BNP natural resources (Harlem et al. 2009), we evaluated threats and stressors on
water resources of BNP attempting to improve understanding of its resources, in order
to inform management of the identified threats directly related to human impacts.
Given the oligotrophic nature of its waters, South Florida’s ecosystems respond
very rapidly to small nutrient enrichment, especially to increases of phosphorous, the
limiting nutrient. Biscayne Bay watershed displays diverse agricultural, urban and
wetland cover, leading to differences in freshwater characteristics as well as in bay
water composition. Median NOx concentrations are higher in agricultural runoff than in
urban or wetland runoff; median concentrations of ammonia, TP, and SRP tend to be
higher in urban runoff than in wetland or agricultural runoff; and median TON
concentrations are higher in wetland and urban runoff than in agricultural runoff.
Legacy and current-use pesticides, herbicides and their metabolites have been
consistently detected, often at low concentrations, in canals and near-shore locations.
Metals display little spatial difference except for lower values near Fisher Island and
relatively high concentrations in sediments near the mouth of the Miami River. Also,
some heavy metals are highest near marinas, where antifouling compounds were also
high. Polychlorinated biphenyls (PCBs) are present in the highest concentrations and 5
times higher in male dolphins with sighting histories in the northern, metropolitan area of
Biscayne Bay than to the south. Polynuclear aromatic hydrocarbons (PAHs) are
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commonly present in sediment samples, at relatively low concentrations with higher
values occurring in marinas.
Climate change threats to Biscayne National Park resources are linked to
changes in rainfall and evaporation patterns affecting the amount of available
freshwater and potentially causing prolonged droughts and/or flooding; saltwater
intrusion into the coastal aquifers and public water supply wells; reduction of coastal
stormwater release capacity due to sea level rise; and changes in tropical storm and
hurricane activity with increased surge levels. One aspect of great concern is the
release of sediment and its associated nutrients and pollutants to the marine system as
coastlines erode due to sea level rise and storm activity.
The Florida Department of Environmental Protection (FDEP) and the US
Environmental Protection Agency (EPA) are in the process of developing numerical
nutrient criteria for estuaries and coastal marine waters for Florida. Given the scarcity
of information on the estuarine effects of nutrient enrichment on South Florida
ecosystem components and the well documented relationship between nutrients and
CHL-a, we approached the derivation of nutrient (Total Nitrogen and Phosphorus)
concentration thresholds for each segment by identifying those concentrations above
which significant increases in CHL-a occur. For this purpose we calculated and plotted
CHL-a z-scored cumulative sum charts (Z-CUSUM) along nutrient gradients, mimicking
cumulative results of nutrient dose-experiments. This allowed us to identify the
successive phytoplankton biomass (CHL-a) reactions to nutrient enrichment, to select
the main threshold and to assess the potential health status of phytoplankton
communities in the water column.
The calculated thresholds fix the upper limit for mean nutrient concentration
above which a mean NNC would not be considered protective of the actual segment’s
baseline conditions with respect to phytoplankton biomass. Hence, we propose the
development of protective nutrient criteria that include only two limits, Long-Term Limit
(LT-L) and Upper Limit (UP-L). The LT-L is the expected mean nutrient concentration in
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a water body at which no adverse effects would be expected or the effects would be
insignificant for the type of designated use. The LT-L is equal to the nutrient threshold.
The UP-L is a nutrient upper bound concentration of the LT-L that accounts for natural
variability in nutrient drivers. The UP-L is the 80th percentile of a normal distribution with
mean equal to the threshold and standard deviation equal to the nutrient’s standard
deviation for the segment under consideration. We suggest the UP-L should not be
breached more than once in a three year period. The selection of the 80th confidence
interval and one- in-three years criteria limits Type I error rates to about 10 percent.
viii
TABLE OF CONTENTS
EXECUTIVE SUMMARY iii
INTRODUCTION 1
SECTION 1: DATA COLLECTION AND QUALITY ASSESSMENT/
QUALITY CONTROL 5
Data Sources 5
Quality Assessment/Quality Control 11
SECTION 2: PARK DESCRIPTION 13
Geology and Soils 15
Water Resources 18
Rainwater 19
Bay waters 24
Biscayne Bay Coastal Wetlands 26
SECTION 3: STATISTICAL ANALYSIS 28