Florida Coastal Everglades Long Term Ecological Research
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PAST WORKING GROUPS

Primary Production Working Group
(Phase II, 2007-2012)


Hypotheses and Proposed Work for FCE II (2006-2012)

General Question: How does a surface water P source versus access to shallow groundwater P affect belowground production and biomass allocation in ecotone plants?

Specific Research Question 1: How does a surface water P source versus access to shallow groundwater P affect belowground production and biomass allocation in ecotone plants?

Soil P availability controls mangrove productivity in the oligohaline ecotone region (Koch & Snedaker 1997; Chen & Twilley 1999). Soil P content in lower Taylor Slough is low at TS/Ph-6 (upper ecotone) and higher at TS/Ph-7 and 8 (lower ecotone; Mancera-Pineda 2003; Chambers & Pederson 2006). However, porewater soluble reactive P (SRP) concentrations from these two sites are similar ( Rivera-Monroy unpubl.data). Since SRP concentrations in surface water are very low (Childers et al. 2006a), it seems likely that groundwater sources may provide P to these shallow peat soils (<1m depth) and thus to the mangroves. Fine root biomass at our TS/Ph ecotone sites is higher than at the SRS sites, suggesting increased plant “foraging” for P, a subsurface P source, or both. Ratios of fine root biomass to aboveground biomass (FRB:ABG) are also greater in sites with the lowest bulk soil P content (Twilley unpubl.data). We also have new isotopic evidence from the TS/Ph ecotone for a strong groundwater influence in aboveground mangrove tissues, compared with no groundwater signal in sawgrass growing at the same site (Ewe, unpubl.data).

Approach - We will continue to measure productivity and biomass allocation at all mangrove sites (SRS-4, 5, & 6 and TS/Ph-6 & 7). We will refine methods for estimating aboveground net primary productivity (ANPP) in the dwarf red mangrove (Rhizophora mangle L.) trees at the TS/Ph 6 & 7 ecotone. These methods are based on regular measurements of leaf turnover, stem elongation, and prop root growth on individual tree clusters. We will also estimate aboveground biomass with annual measurements of crown area and prop root number from 16 randomly selected tree clusters at each site using allometric equations (Coronado-Molina et al. 2004). We will continue to quantify belowground biomass and production using standard coring and in-growth core techniques.

In Year 2, we will initiate a small scale fertilization experiment to better understand how P availability controls dwarf R. mangle productivity and biomass allocation. Work by Feller (1995), Feller et al. (1999), and Lovelock et al. (2004) has shown dramatic effects of nutrient addition on dwarf R. mangle growth, insect herbivory, mangrove physiology, and nutrient-use efficiency. We will select 16 tree clusters at the TS/Ph-6 ecotone site. Of those, half will receive a P-amended in-growth root core while the other half will receive in-growth cores without added P. In January, May, and September, we will collect all in-growth cores and replace each with a new core of the same treatment. Following protocols described above, we will track each experimental tree cluster for 2 years. These data will provide important insights into the seasonality of biomass allocation as modified by P availability. This experiment will also provide valuable pilot data that we will use to develop larger, ecosystem- scale nutrient manipulations based on our growing knowledge of how surface versus subsurface supplies of P affect mangrove ecosystem function.


Specific Research Question 2: How does a surface water P source versus access to shallow groundwater P affect sawgrass and periphyton productivity at the freshwater- ecotone transition? How does increased hydroperiod and decreased water residence time (as a result of increased freshwater inflows) affect productivity in this transition zone?

Primary production at freshwater FCE sites is dominated by sawgrass and periphyton, with highest rates of turnover occurring near the oligohaline ecotone, particularly along the TS/Ph transect (Ewe et al. 2006; Iwaniec et al. 2006; Fig. 2-15). In SRS, where hydroperiods are relatively longer and water residence times shorter (compared to TS/Ph), sawgrass production ranges from 300-600 g C m-2 yr-1 (Ewe et al. 2006) while periphyton production ranges from 17-60 g C m-2 yr-1 (Gaiser unpubl.data). Both of these values increase toward the ecotone, where marine P subsidizes production—particularly during the dry season. In TS/Ph, sawgrass production is lower (250-400 g C m-2 yr-1; Childers et al. 2006b; Ewe et al. 2006) while periphyton production is one to two orders of magnitude higher (300-18000 g C m-2 yr-1; Iwaniec et al. 2006); again, with values increasing toward the ecotone. Along both transects, we expect that higher productivity near and in the ecotone is driven by enhanced P availability relative to upstream freshwater marshes.

Increased freshwater flow should increase hydroperiod and water depth and may decrease the importance of groundwater inputs. We expect that these hydrologic changes will lead to a decline in sawgrass ANPP (Childers et al. 2006b) and a shift from soil-associated periphyton communities (where P supply is relatively enhanced) to water column or plant-associated communities (where P supply is relatively depleted). Because we expect that increased freshwater inflows will enhance oligotrophy in the ecotone regions, we expect an associated reduction in periphyton productivity (Gaiser et al. 2006) in these areas.

Approach - We will continue to measure sawgrass ANPP and periphyton productivity at freshwater FCE sites as per Childers et al. (2006b) and Iwaniec et al. (2006), with modifications suggested by Ewe et al. (2006). Specifically, we have found that quantifying periphyton productivity requires a variety of approaches, including biomass accumulation on artificial substrates and light-dark bottle biological oxygen demand (BOD), but with added emphasis on the latter method because it generates the lowest variance in estimates (Hall et al. 2006).
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National Science Foundation logo This material is based upon work supported by the National Science Foundation through the Florida Coastal Everglades Long-Term Ecological Research program under Cooperative Agreements #DEB-1237517, #DBI-0620409, and #DEB-9910514. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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