Christopher J. Madden

Importance of Storm Events in Controlling Ecosystem Structure and Function in a Florida Gulf Coast Estuary

Abstract From 8/95 to 2/01, we investigated the ecological effects of intra- and inter-annual variability in freshwater flow through Taylor Creek in southeastern Everglades National Park. Continuous monitoring and intensive sampling studies overlapped with an array of pulsed weather events that impacted physical, chemical, and biological attributes of this region. We quantified the effects of three events representing a range of characteristics (duration, amount of precipitation, storm intensity, wind direction) on the hydraulic connectivity, nutrient and sediment dynamics, and vegetation structure of the SE Everglades estuarine ecotone. These events included a strong winter storm in November 1996, Tropical Storm Harvey in September 1999, and Hurricane Irene in October 1999. Continuous hydrologic and daily water sample data were used to examine the effects of these events on the physical forcing and quality of water in Taylor Creek. A high resolution, flow-through sampling and mapping approach was used to characterize water quality in the adjacent bay. To understand the effects of these events on vegetation communities, we measured mangrove litter production and estimated seagrass cover in the bay at monthly intervals. We also quantified sediment deposition associated with Hurricane Irenes flood surge along the Buttonwood Ridge. These three events resulted in dramatic changes in surface water movement and chemistry in Taylor Creek and adjacent regions of Florida Bay as well as increased mangrove litterfall and flood surge scouring of seagrass beds. Up to 5 cm of bay-derived mud was deposited along the ridge adjacent to the creek in this single pulsed event. These short-term events can account for a substantial proportion of the annual flux of freshwater and materials between the mangrove zone and Florida Bay. Our findings shed light on the capacity of these storm events, especially when in succession, to have far reaching and long lasting effects on coastal ecosystems such as the estuarine ecotone of the SE Everglades.

Sediment-water oxygen and nutrient fluxes in a river-dominated estuary

Sediment oxygen uptake and net sediment-water fluxes of dissolved inorganic and organic nitrogen and phosphorus were measured at two sites in Fourleague Bay, Louisiana, from August 1981, through May 1982. This estuary is an extension of Atchafalaya Bay which receives high discharge and nutrient loading from the Atchafalaya River. Sediment O2 uptake averaged 49 mg m−2 h−1. On the average, ammonium (NH4+) was released from the sediments (mean flux =+129 μmol m−2 h−1), and NO3− was taken up (mean flux =−19 μmol m−2h−1). However, very different NO3− fluxes were observed at the two sites, with sediment uptake at the upper, river-influenced, high NO3− site (mean flux =−112 μmol m−2 h−1) and release at the lower, marine-influenced low NO3− site (mean flux =+79 μmol m−2 h−1). PO43− fluxes were low and often negative (mean flux =−8 μmol m−2 h−1), while dissolved organic phosphorus fluxes were high and positive (mean flux =+124 μmol m−2 h−1). Dissolved organic nitrogen fluxes varied greatly, ranging from a mean of +305 μmol m−2 h−1 at the lower bay, to −710 μmol m−2 h−1 at the upper bay. Total dissolved nitrogen and phosphorus fluxes indicated the sediments were a nitrogen (mean flux =+543 μmol m−2 h−1) and phosphorus source (mean flux =+30 μmol m−2 h−1) at the lower bay, and a nitrogen sink (mean flux =−553 μmol m−2 h−1) and phosphorus source (mean flux =+17 μmol m−2 h−1) in the upper bay. Mean annual O∶N ration of the positive inorganic sediment fluxes were 27∶1 at the upper bay and 18∶1 at the lower bay. Based on these data we hypothesize that nitrification and denitrification are important sediment processes in the upper bay. We further hypothesize that Atchafalaya River discharge affects sediment-water fluxes through seasonally high nutrient loading which leads to net nutrient uptake by sediments in the upper bay and release in the lower bay, where there is less river influnces.

An instrument system for high-speed mapping of chlorophyll a and physico-chemical variables in surface waters

A device incorporating microprocessor control and flow-through sampling permits high-speed measurements of chlorophyll a, and physical and chemical variables in aquatic systems. Continuous sampling of chlorophyll a, conductivity, temperature, salinity, incident photosynthetically active radiation (PAR), underwater PAR, and pH facilitates multiple correlations and mapping of variables at both small and large spatial scales. The instrument is portable and can be installed in a small boat for “rapid response” sampling of large areas. The low-voltage DC power requirement and shallow draft make the device especially suitable for work in shallow coastal areas, tidal creeks, bayous, and physically complex aquatic landscapes where larger vessels cannot be operated. Examples of applications of the instrument are discussed. In Fourleague Bay, Louisiana, a shallow estuary on the Gulf of Mexico, we were able to detect horizontal chlorophyll a structure and transient fronts map spatial variations on the scale of a few meters to several kilometers, and follow movements of chlorophyll features through the estuary. These patterns were often not apparent when sampled at discrete stations.