Biogeochemistry of Macroalgal-Derived Floc Layer: Examination of an apparent nutrient source to mangroves bordering shallow ponds at Twin Cays, BZ


Stephen E. Davis, III and Ilka C. Feller


The dwarf mangroves at Twin Cays show evidence of a natural fertilization effect from an allochthonous source of nutrients that appears to be shaping forest structure and productivity along the windward edge of shallow mangrove ponds. Prevailing winds across Twin Cays continually generate large, thick deposits of Batophora detritus along the windward edge of these ponds. As this accumulated algal material degrades, it forms an unconsolidated layer of floc at the sediment surface and seems to correspond with the greater tree height and internode lengths exhibited by the mangroves growing in this zone. This phenomenon has led to the hypothesis that the accumulation of Batophora detritus has a natural “fertilization effect” on the mangroves at the receiving end of this floc. In this study, we are investigating the biogeochemistry of this floc material to better understand how it controls mangrove forest development. This study involves field sampling and experiments to answer the following questions: 1) How does the physico-chemical properties within the floc layer differ from nearby bare sediment? 2) How much nitrogen and phosphorus is released from through short-term decay of Batophora? 3) Do mangroves growing within the floc zone derive a portion of their nutrition from the floc? 4) What is the importance of this material in driving ecosystem function? We expect that the decay of Batophora detritus will yield a substantial short-term source of nitrogen and phosphorus as has been shown for other algal species. Using stable isotope signatures and analyses of root biomass, we further hope to establish this material as a source of available nutrients (especially N) to mangroves. This work will add to our knowledge of the relative importance of autochthonous versus allochthonous detritus in regulating mangrove ecosystem structure and function in these enclosed island mangrove systems. .
 

Spartina alterniflora leaching work at North Inlet


Stephen E. Davis, III

In order to understand the short-term fate of macrophyte detritus in a N-limited coastal wetland in North Inlet, SC, I conducted studies to quantify the rapid (hours to days) loss of nutrients (N and P) and dissolved organic carbon from senesced culms of Spartina alterniflora. These studies were carried out as short-term (21-day) incubations of macrophyte tissue in sealed, glass containers with ambient surface water. This work was supported through a 2002 Visiting Scientist award from the Baruch Foundation (Univ. South Carolina). Half of the samples received a poison to remove any biological effect on leaching. At each sampling (0 hours, 24 hours, 48 hours, 5 days, 10 days, and 21 days), water samples were collected and later analyzed for Total Phosphorus, Total Nitrogen, and Total Organic Carbon. Short-term decay was modeled with and without the influence of microbes and nutrient release indicated a significant release of TP, TN, and TOC over a 21-day period as a result of leaching. These results followed a similar pattern exhibited by macrophyte species from the southern Everglades (FL) and will be used as evidence to support macrophyte leaching as a rapid, significant source of limiting nutrients in wetland ecosystems.

Macrophyte Litter in the Southern Everglades:A rapid and substantial source of energy and nutrients to the detritus food chain


Stephen E. Davis, III and Daniel L. Childers


Dissolved organic matter (DOM) derived from senesced macrophytes is an important source of energy and nutrients in oligotrophic wetlands such as southern Everglades, where concentrations of inorganic P are near detection limits and organic N and P are oftentimes 100-fold greater than inorganic N and P. To examine the contribution of leaf litter to DOM loads in the water column of this region, we conducted short-term decomposition experiments of leaves from three common macrophytes: red mangrove, sawgrass, and spikerush. Leaves were decomposed in bottles with ambient water, half of which were poisoned. Water samples were collected after 0, 1, 2, 5, 10, and 21 days to determine the contribution of leaves to organic carbon (OC), N and P loads. Our findings indicated a significant microbial effect on OC and P leaching from all species, with higher leaching rates in the poisoned samples. Leached OC in the non-poisoned bottles was rapidly respired and P was retranslocated back to the surface of the leaves in the form of microbial biomass. Leaching rates of OC from mangrove and sawgrass were an order of magnitude greater than those for spikerush. Leaching rates of TP from sawgrass and spikerush were 3-fold greater than mangrove in the poisoned samples, but all were similar in the non-poisoned bottles. The molar ratios of N:P flux from the litter ranged from 2-10. Compared with typical molar ratios of N:P in the surface water (≈150), litter leaching seems to represent a substantial, rapid source of P to this region.

Organic Carbon Flux at the Mangrove Soil-Water Column Interface in the Florida Coastal Everglades


Melissa Romigh, Robert Twilley, Stephen E. Davis, II, and Victor Rivera-Monroy


Coastal outwelling of organic carbon from mangrove wetlands contributes to near-shore productivity and influences biogeochemical cycling of elements. We used a flume to measure fluxes of dissolved organic carbon (DOC) between a mangrove forest and adjacent tidal creek along Shark River, Florida. Shark River’s hydrology is influenced by diurnal tides and seasonal rainfall and wind. Samplings were made over multiple tidal cycles in 2003 to include dry, wet, and transitional seasons. Surface water [DOC], temperature, salinity, conductivity and pH were significantly different among all sampling periods. [DOC] was highest during the dry season (May), followed by the wet (October) and transitional (December) seasons. Net DOC export was measured in October and December, inferring the mangrove forest is a source of DOC to the adjacent tidal creek during these periods. This trend may be explained by high rates of rainfall, freshwater inflow, and subsequent flushing of wetland soils during this period of the year.

Tidal Creek Fluxes of Materials Along the Guadalupe Estuary (TX)


Bryan Allison and Stephen E. Davis, III


Freshwater inflow is an important part of the subsidy and maintenance of estuarine ecosystems. This is especially true in the Guadalupe Estuary, where estuarine marshes support the last migrating population of whooping cranes during the winter. A proposed diversion of freshwater from the lower Guadalupe River led to a study of the factors affecting marsh ecosystem structure and function, particularly as they relate to this endangered species. In order to predict the diversion’s impact, it is imperative to understand the factors governing landscape patterns of salinity, inundation, and macrophyte community structure. These patterns are driven to some extent by the exchange of materials between upland, marsh, and adjacent estuarine waters. The primary objective of this research is to quantify suspended sediment and floc exchange in three tidal creeks in response to natural variations in tides, riverine inflows, wind forcing, and barge traffic. Our preliminary data indicate these natural and anthropogenic forces are all important in regulating salinity patterns, inundation regimes, and exchange of materials, but over dramatically different time scales. Barges affect creek hydrodynamics and can mobilize sediment on scales of minutes. Wind forcing operates over slightly longer time scales (hours). Tides operate weakly over diurnal time scales, but to a greater extent over fortnightly and semi-annual scales. Finally, freshwater inflows are more difficult to assess, as they vary widely across space and time. However, continued sampling over an array of inflow conditions over multiple years will help elucidate the role of riverine inflows in materials exchange.

Seasonal variation of productivity and respiration in a tropical blackwater river: The role of allochthonous organic matter and inorganic nutrients


Daniel L. Roelke, Carlos del Castillo, Stephen E. Davis, III, Jose-Vicente Montoya, Kirk O. Winemiller, James Cotner.

The Cinaruco River is a blackwater ecosystem in the Venezuelan llanos (savannah). The river has strong seasonal hydrology and supports large populations of ecologically diverse fishes. Undetermined are the relative contributions of autochthonous (aquatic) and allochthonous (terrestrial) production sources supporting high stocks of secondary consumers. Using excitation-emission fluorescence spectroscopy and absorption spectra of colored dissolved organic matter we were able to infer degradation of leaf material originating from the surrounding gallery forest. Our results suggest that during the low-water period a large fraction of fluorescent organic matter contained in leaves was degraded quickly in river water. This represented a prevalent allochthonous contribution of carbon into the system at this time. During the falling water period, however, the contribution of organic matter from trees was much less. Furthermore, the fluorescence signature of dissolved organic matter in lagoons was different from that of the main river channel, which suggests that organic matter originated from the savannah at this time. Despite this clear allochthonous organic matter source fueling microbial respiration, our in-water experiments using light and dark incubator technology indicated that autochthonous productivity was an important supplementary carbon source, perhaps fueling higher trophic levels. During the low-water and falling-water periods, water column primary productivity in the euphotic zone ranged from 150 to 500 mg-C m-2 d-1, 2-fold greater than respiration. Similarly, along the river edges, where light penetration reached the sediments, benthic primary productivity was on the same order as respiration, i.e., ranging between 350 and 500 mg-C m-2 d-1. Primary production was likely limited by the availability of dissolved inorganic nitrogen, which ranged from below detection limits to 0.5 µM, with averages typically of ~0.25 µM.