An Assessment of Nearshore Benthic Communities of the Florida Keys
Leanne Miller Rutten, Jim Fourqurean, and Tom Philippi
Florida International University, Miami, FL
Understanding the causes of degradation to coastal marine ecosystems in the
Florida Keys is a significant research and management challenge. An important
aspect of this challenge is distinguishing between influences of natural
environmental factors and anthropogenic factors on spatial and temporal
variations in nearshore environments. Anthropogenic disturbance is often claimed
to be the primary factor responsible for degradation of coastal marine ecosystems
in the Florida Keys, however adequate scientific data are lacking that support this
conclusion. Further, natural system variability caused by large storms, climatic
shifts, and geologic history also contributes to changes within coastal marine
ecosystems. Answers to the fundamental questions about causes of environmental
changes in the Florida Keys remain elusive because few studies address
ecosystem condition in different habitat types, at multiple scales, and over large
spatial and temporal extents. Offshore coral reefs in the Florida Keys have
received the most attention in terms of the numbers of studies conducted, but
considerable debate still surrounds their condition. Interestingly, nearshore
habitats, which are closest the potential sources of local anthropogenic
disturbance and most likely to exhibit signs of anthropogenic disturbance, have
been largely overlooked. In response to this oversight, a landscape scale
investigation of nearshore (<1 km from shore) benthic communities of the Florida
Keys was initiated.
Study areas included four regions within the Florida Keys: Key Largo, Marathon,
Big Pine, and Key West. The study areas enabled sampling efforts to be
distributed across the geographic extent of the keys while accounting for
ecosystem variability. These four study areas were selected because they
comprised the wide range of nearshore and terrestrial communities, as well as
human populations, found in the Florida Keys. The study areas were characterized
by a latitudinal extent of nearly 100 km and a longitudinal extent exceeding 160
km. Sampling efforts were concentrated in the nearshore zone for two reasons.
First, little nearshore research has been conducted in the Florida Keys. The data
collected extended existing offshore benthic survey and nutrient sampling
programs to the nearshore environment. Second, if nutrients (including
anthropogenically derived wastewater) are transported via groundwater or runoff
in the Florida Keys, the nearshore zone is the one area where the effects may be
detectable. However, it is known that excessive nutrient enrichment may only
have localized effects in nearshore environments. Thus, sampling efforts within
the study areas were distributed to include areas of heavy coastal development as
well as undeveloped coastline. In addition, all sampling efforts were spatially
intensive (on the order of 100s of meters) and concentrated close to the shoreline.
Part one of the investigation was designed to 1) document the current composition
and distribution of nearshore benthic communities and 2) expose environmental
factors (e.g. water depth, substrate type, sediment depth) affecting spatial
variations in nearshore benthic communities. Benthic surveys were used to obtain
comprehensive data on nearshore benthic community composition. A consensus
classification of the community composition data resulted in eight nearshore
benthic community classes; five classes represented seagrass communities, and
the remaining three classes comprised hardbottom communities. The distribution
of nearshore benthic community classes revealed spatially coherent variation of
community composition within the study areas. There were also significant
differences in the distribution of nearshore benthic community classes among
study areas throughout the Florida Keys. Discriminant function analysis revealed
that environmental factors could predict nearshore benthic community class
membership with accuracies ranging from 25 to 50 percent.
Part two of the investigation was designed to evaluate relationships between
human land use activity and the composition of nearshore benthic communities.
In order to conduct the evaluation, models were developed that incorporated
nearshore benthic survey data, natural environmental factors (water depth,
substrate type, and sediment depth), and human land use activity (categorical
land use data, quantitative nutrient loading data). GIS technology was used to
calculate metrics that represented the influence of land use and nutrient loading on
nearshore benthic habitats. The composition of nearshore benthic communities,
represented by benthic taxa densities, was significantly influenced by proximity to
land, water depth, substrate type and sediment depth. Many benthic taxa densities
were also significantly influenced by human land use activity. However, these
results varied tremendously, depending on which metric of human land use
activity was used in the model.
Part three of the investigation was designed to describe spatial variations in the
relative availability of nutrients in nearshore benthic environments. In addition,
the relationship between nearshore nutrient availability and adjacent land use or
nutrient loading was explored. Based upon the elemental content and ratios of the
seagrass Thalassia testudinum, nearshore environments of the Florida Keys were
characterized by low phosphorous availability. Nearly all response variables
included in this investigation (seagrass, epiphyte, and sediment) exhibited
significant spatial variation with respect to study area (Key Largo, Marathon, Big
Pine, and Key West) and location (bayside or oceanside), but little significant
spatial variation with respect to either distance from shore or land use.
Part four of the investigation was designed to characterize changes in nearshore
benthic macrophytes to provide long-term perspective on historical changes in
nearshore benthic communities. Specifically, the study was designed to 1)
identify temporal variations in nearshore benthic macrophytes and 2) determine if
these variations may be associated with land use in the Florida Keys. The
analyses were conducted using four decades of black and white aerial
photographs taken by the Florida Department of Transportation (FDOT). In
general, nearshore benthic macrophyte communities in the Florida Keys displayed
tremendous stability between 1959 and 1998; nearshore benthic macrophyte
communities in the Florida Keys exhibited very slight positive changes overall.
There were small, but significant differences in the amounts of change in
nearshore benthic macrophytes among some of the five time steps (each step
represented approximately 8 years). The nature of these small temporal changes
in nearshore benthic macrophytes exhibited some spatial variation. The
magnitude and direction of changes in benthic macrophytes were larger and more
positive (net macrophyte increases) in the Key Largo and Marathon study areas
but smaller and more negative (net macrophyte decreases) in the Big Pine and
Key West study area changes. Human land use activity was shown to have little
effect on changes in nearshore benthic macrophytes in the Florida Keys.
Leanne Miller Rutten, Florida International University, University Park, Miami,
FL, 33199, Phone: (305) 348-6997, Fax: (305) 348-4096, Email: lrutte01@fiu.edu