Figure 1. Pie charts of water budget components in Shark and Taylor Sloughs. Inputs include rain (blue), surface water inflow (red), and groundwater discharge (green). Outputs consist of evapotranspiration (ET, blue), surface water outflow (red), and groundwater seepage (green). Values represent the average percentages of each component for Taylor Slough on a monthly basis between Jan. 2008 - July 2009, and on an annual basis in Shark Slough between 2002-2008.
FCE hydrological research in the estuarine ecotone provided the first evidence for significant landward movement of brackish water from the Gulf of Mexico into the coastal zone. This brackish groundwater discharge not only delivers saline water to the interior, but also delivers associated phosphorus which stimulates productivity there. This groundwater delivery of phosphorus is especially visible in the Taylor River drainage during the dry season.
The main goals of the hydrology group were to quantify the major water balance parameters: freshwater inflow from the upstream Everglades; marine water; groundwater, precipitation and evapotranspiration in both Shark and Taylor Sloughs. Groundwater discharge was investigated in terms of surface water quality as well as its potential effects on ecological process. In addition, the role of water management and sealevel rise on water levels and residence times were investigated. Our results indicate that in each slough, precipitation was the dominant water source while evpotranspiration was the dominant water loss. Groundwater inputs were found to be a significant contribution to the surface water in both Taylor and Shark Sloughs (Fig. 1). In Taylor Slough, groundwater inputs were the second largest input, accounting for about 25% of the total inputs, with upstream freshwater flows accounting for only 8% of the surface water. In Shark Slough, groundwater input was equivalent in magnitude to the upstream freshwater flows across Tamiami Trail, representing 19% of the total. Groundwater inputs are important for the oligohaline ecotone region of the FCE as both fresh and brackish groundwater contains higher concentrations of phosphorus, the limiting nutrient within our ecosystem. Marine waters from Florida Bay and the Gulf of Mexico intrude into the coastal aquifer beneath the oligohaline ecotone, causing the groundwater to be brackish.
Figure 2. Monthly Groundwater discharge to SRS and monthly surface water salinity at SH2 (the oligohaline ecotone) averaged over 2003-2008 (Saha et al., 2012).
Our investigations have determined that brackish groundwater discharge (GWD) combined with long residence times and evaporation had a greater effect on water chemistry in Taylor Slough which seasonally became hypersaline as compared to Shark Slough which only became moderately hypersaline. The highest values of GWD occurred in May-July (Fig. 2) concurrent with the highest levels of evapotranspiration, solar radiation, phosphorus concentrations, and hypersalinity conditions. Ecosystem response was mixed under these conditions, with mangrove physiological functions limited but gross primary production becoming.
The position of the groundwater mixing zone varies seasonally but has been steadily increasing inland with sealevel rise. Water levels along the coastline have been increasing since the 1960s at a rate of 2 mm/yr, equivalent to local sealevel rise (Fig. 3a). While, water levels in the upper reaches of Shark Slough have increased at a higher rate (7mm/yr) due to increased releases of fresh surface water across Tamiami Trial (Fig. 3b). Despite the increased releases of freshwater from the upstream Everglades, the resultant water becomes ponded in the upper reaches of Shark Slough due to the presence of sawgrass and floating peryphyton mats within the water column which restrict flow velocities. Our results suggest that managed surface water inflows to Shark Slough need to be increased in order to counteract the landward intrusion of seawater. Estimated residence times in Taylor and Shark Sloughs are variable. Preliminary estimates of residence times in Taylor Slough indicate a bi-modal distribution with peaks at 7 and 180 days, corresponding with periods of high flow and low flow, respectively. In Shark Slough, residence times decreased in a downstream direction from >90 days to about 14 days between SRS-1 and SRS-4 (Fig. 3), due to increased surface water flow velocities in the slough. Within the mangrove ecotone region of Shark River Slough, where tides occur, residence times of 12 days or less were estimated.