Macrophyte Benthic Communities and Groundwater Nutrient Dynamics in Biscayne Bay,
Florida
Danielle Mir-Gonzalez, and Joseph N. Boyer
Southeast Environmental Research Center, Florida International University, Miami FL
Most researchers agree that anthropogenic nutrient loading may affect the structure and
function of benthic communities in shallow, marine systems. One important source of
nutrient loading may be groundwater, especially in karstic limestone systems. The effects
of groundwater on benthic vegetation are not well understood in estuarine areas and it
may have a great impact on water quality. The presence of two landfills (old and new),
and the large agricultural centers in southeast Miami pose a definite threat of
groundwater quality, making this an interesting area to study.
Unlike the nearby estuary, Florida Bay, southern Biscayne Bay has very little background
information on seagrass community structure and most of the published literature dates
from the 1960’s and 1970’s. The distribution of Thalassia testudinum and other aquatic
vegetation has been linked to groundwater discharges in Biscayne Bay (Kohout and
Kolipinski 1967; Meeder et al. 1997).
This investigation is focused on the southwest region of Biscayne Bay, between Black
Point and Turkey Point, and has found spatial variations of the nearshore (< 1 km)
benthic macrophyte communities. Four transects consisting of four sites each were
chosen based on the potential of groundwater and canal run-off nutrient concentrations.
Seepage meters were installed along these transects to quantify potential nutrient loading
of groundwater with distance from shore. We measured groundwater flow rates, nutrient
concentrations in groundwater and surface water, and benthic community structure at
each site.
Average seepage flux highs varied between transects (116 L m-2 day-1 to 13 L m-2 day-1)
during the months of August, October and December 2002. The highest flow rates were
found near Mowry Canal 150 m offshore with Black Point having the second highest 300
m from shore. In general, the water obtained from seepage meters was significantly
higher in phosphorus, total nitrogen, organic carbon, and ammonia (NH4+). Densities of
T. testudinum plummet to zero in the Black Point vicinity. A “snap-shot” study done by
Meeder & Boyer (2001) showed a similar pattern, where high levels of NH4+
concentrations at Black Point was correlated with an absence of T. testudinum. However,
the nutrient levels may mask other factors that may be responsible for the decline such as
salinity variations (Meeder, et. al, 2000). Ruppia maritima and Chara sp. were found
where T. testudinum was absent. Halodule wrightii was the dominant species close to
shore (<500 m) and in the Black Point area. The variations within the benthic
macrophyte habitat will continue to be investigated in relation to nutrient availability, and
groundwater inputs.
Surveyed March- May 2002
Literature Cited:
Kohout, F.A., Kolipinski, M.C. 1967. Biological Zonation Related to Groundwater
Discharge Along the Shore of Biscayne Bay, Miami, Florida, Estuaries: American
Association for the Advancement of Science, pp 488-499.
Meeder, J., Boyer, J.N. 2001 Total Ammonia Concentrations in soil, sediments, surface
water, and groundwater along the western shoreline of Biscayne Bay with the focus on
Black Point and a reference mangrove site. BISC-N-011.000. Final Report to National
Park Service, SERC, FIU, Miami.
Meeder, J.F., Alvord, J., Byrnes, M., Ross, M.S., Renshaw, A. 1997 Distribution of
benthic nearshore communities and their relationship to groundwater nutrient loading.
BNP. Final Report to Biscayne National Park, SERC, FIU, Miami.
Southeast Environmental Research Center, Florida International University, University
Park- OE 148, Miami, FL 33199. Phone: 305-348-3594,
Fax: 305-348-6202, dmir01@fiu.edu, Question # 4