Historic primary producer communities linked to water quality and hydrologic changes in the northern Everglades
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- Waters, M.N., Smoak, J.M. & Saunders, C.J. J Paleolimnol (2013) 49: 67. doi:10.1007/s10933-011-9569-y
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The northern Everglades Water Conservation Areas have experienced recent ecological shifts in primary producer community structure involving marl periphyton mats and dense Typha-dominated macrophyte stands. Multiple investigations have identified phosphorus (P) as a driver of primary producer community structure, but effects of water impoundment beginning in the 1950s and changes in water hardness [e.g., (CaCO3)] have also been identified as a concern. In an effort to understand pre-1950, primary producer community structure and identify community shifts since 1950, we measured pigment proxies on three sediment cores collected in Water Conservation Area-2A (WCA-2A) along a phosphorus enrichment gradient. Photosynthetic pigments, sediment total phosphorus content (TP), organic matter, total organic carbon and nitrogen were used to infer historic primary producer communities and changes in water quality and hydrology regulating those communities. Excess 210Pb was used to establish historic dates for the sediment cores. Results indicate the northern area of WCA-2A increased marl deposition and increased algal abundance ca. 1920. This increase in (presumably) calcareous periphyton before intensive agriculture and impoundment suggest canal-derived calcium inputs and to some extent early drainage effects played a role in initiating this community shift. The northern area community then shifted to Typha dominance around 1965. The areas to the south in WCA-2A experienced increased marl deposition and algal abundance around or just prior to 1950s impoundment, the precise timing limited by core age resolution. Continued increases in algal abundance were evident after 1950, coinciding with impoundment and deepening of canals draining into WCA-2A, both likely increasing water mineral and nutrient concentrations. The intermediate site developed a Typha-dominated community ca. 1995 while the southern-most core site WCA-2A has yet to develop Typha dominance. Numerous studies link sediment TP >650 mg P/kg to marsh habitat degradation into Typha-dominance. The northern and intermediate cores where Typha is currently support this previous research by showing a distinct shift in the sediment record to Typha dominance corresponding to sediment TP between 600 and 700 mg P/kg. These temporal and spatial differences are consistent with modern evidence showing water-column gradients in mineral inputs (including Ca, carbonates, and phosphorus) altering primary producer community structure in WCA-2A, but also suggest hydroperiod has an effect on the mechanisms regulating periphyton development and Typha dominance.
KeywordsFlorida EvergladesPaleoecologyPhotosynthetic pigmentsPhosphorusMarl
The Florida Everglades is one of the largest freshwater wetlands in the world. Beginning in the late 1800s to the 1950s, agricultural practices began to increase around the Everglades, and canals were constructed in an effort to regulate water and flow. In the 1950s and 1960s, the northern Everglades water flow was contained and redirected into Water Conservation Areas (WCA) by the Army Corps of Engineers to allow development, construction and agriculture within and surrounding the Everglades (USACE 1999). The change in hydrology reduced water flow to Florida Bay, and the Everglades decreased to half its original area with a substantial decrease in water quality (McCormick et al. 2002). Agricultural runoff and urban stormwater have introduced phosphorus (P) and other contaminants into the Everglades, degrading aquatic habitats. Enrichment of Everglades’ surface water with P is of concern because P inputs have been documented to dramatically change the ecological structure and function of Everglades wetland communities (Newman et al. 1996; McCormick et al. 2001; Gaiser et al. 2005). In addition, changes in water hardness (mg CaCO3 l−1) associated with canal-inputs have impacted the potential for P sequestration in marsh soil (Vaithiyanathan and Richardson 1997) and periphyton structure (Browder et al. 1994).
The primary producer community structure of the Everglades is affected by nutrient loading. In response to increased phosphorus and water hardness, extensive marl periphyton mats have developed over much of the Everglades (Gaiser et al. 2004). The periphytic mats can be attached to macrophyte communities, benthic or free floating and have been shown to shift from diatom/cyanobacterial dominance to chlorophyte dominance in response to increasing phosphorus loads (McCormick et al. 1998). Recent concerns in primary producer community structure in WCA-2A have focused on the development of dense Typha communities (Noe et al. 2002). Typha grows rapidly and overtakes periphyton and other macrophytes forming a monoculture highly resilient to perturbations (Gunderson 2001; Hagerthey et al. 2008). Previous analyses of siliceous microfossil profiles in enriched and unenriched areas of WCA-2A have shown significant changes in the primary producer community composition since 1950, which coincides with the timing of canal-derived water inputs (Slate and Stevenson 2000), but little is known of the primary producer community structure prior to 1950.
Description of pigments measured, abbreviations and diagnostic application
Total primary producer abundance
Total primary producer abundance
Total primary producer abundance
Lutein + Zeaxanthin
Lut + Zea
Chlorophyta + Cyanobacteria
Cyanobacteria (attached, colonial)
Cyanobacteria (attached, colonial)
Cyanobacteria (UV-blocking pigment)
Core sites, descriptions and locations of coring sites in WCA-2A
Unenriched, periphyton dominated
Moderately enriched, Typha dominated
Enriched, densely Typha dominated
Cores were extruded and sliced at 1 cm intervals to 26 cm depth, at which point the interval depth was increased to 2 cm for the remainder of the core. A portion of the wet material was separated for gravimetric analysis. Gravimetric analysis included wet and dry weights and loss on ignition at 550°C. The remaining sample material was freeze dried and ground into fine powder with a mortar and pestle for 210Pb, photosynthetic pigments, total phosphorus, organic carbon and nitrogen analysis.
Lead-210 and 226Ra measurements were made using an intrinsic germanium detector coupled to a multi-channel analyzer. Freeze dried and ground sediments were packed and sealed in gamma tubes. Activities were calculated by multiplying the counts per minute by a factor that includes the gamma-ray intensity and detector efficiency. This factor was determined from standard calibrations. Identical geometry was used for all samples. Lead-210 activity was determined by the direct measurement of 46.5 keV gamma peak. Radium-226 activity was determined from 214Pb energy at 351.9 (Moore 1984). For 226Ra measurements, the sealed samples were set aside for at least 21 days to allow for 222Rn to ingrow and secular equilibrium to be established between 226Ra and its granddaughter 214Pb. Excess 210Pb activity was calculated by subtracting the supported 210Pb (i.e., 226Ra activity) from the total 210Pb activity. Excess 210Pb and the sediment mass in each interval were used in the Constant Rate of Supply (CRS) model to determine mass accumulation rates and ages of sediment intervals (Appleby 2001).
Photosynthetic pigments (chlorophylls and carotenoids) were measured using an HPLC system following the methods of Leavitt and Hodgson (2001) designed particularly for sedimentary pigments. Dried sediment samples were extracted with a solvent mixture of acetone, methanol and water mixed in an 80/15/5 ratio, which contained an internal standard (Sudan II; Sigma Chemical Corp., St. Louis, MO, USA) and allowed to digest 16–24 h in a −20°C freezer. Following extraction, samples were centrifuged and filtered through a 0.22 μm syringe filter to remove particulate matter. Samples were placed in an auto-sampler tray where they were mixed with an ion-pairing agent (0.75 g tetrabutylammonium acetate and 7.7 g ammonium acetate in 100 ml HPLC-grade water) prior to injection. 200 μl of each sample was injected into a Shimadzu HPLC system following the mobile phase and time sequence of Leavitt and Hodgson (2001). Chlorophylls and carotenoids were separated by passing through a Rainin Model 200 Microsorb C18 column and measured using a photodiode array detector set at 435 and 665 nm. Pigment identification was made using retention times of known standards and pigment specific spectra recorded by the detector. Pigment concentrations are expressed as nmol pigment/g OM and calculated by comparing peak areas against standards of known concentration.
For total phosphorus analysis, sediment subsamples were digested with H2SO4 and K2S2O8 and measured using an autoanlyzer with a single channel colorimeter. For total carbon and nitrogen analyses of sediment organic matter, sediment subsamples were acidified with diluted HCl to remove carbonate and washed with copious amounts of deionized water. Samples were analyzed using a CHN analyzer.
Core descriptions and chronology
The southern-most core, EV-1 contained a marl layer that extended from the top of the core down to 10 cm. Below this layer the core contained dark organic peat. The two northern most cores, EV-2 and EV-3, contained flocculent organic material from the top of the core to 9 and 23 cm, respectively. Below this layer, a distinct marl layer occurred from 9–14 to 23–30 cm for EV-2 and EV-3, respectively. Below the marl layer, both cores contained dark organic peat.
Organic proxy measurements and phosphorus
Prior research has identified phosphorus and hydrology as drivers of primary producer community structure in the Everglades (Browder et al. 1994; McCormick et al. 2002; Noe et al. 2001; Ross et al. 2003). Before extensive modifications began in the Everglades (pre-1917) phosphorus limited primary producer growth and entered the system through precipitation. Due to the building of canals and intense agriculture, phosphorus and other mineral inputs into WCA-2A increased, causing changes in the primary producer community structure from diverse macrophyte and epiphytic communities to dense macrophyte stands dominated by Typha (Hagerthey et al. 2008). In this study, paleolimnological proxies indicate three historic trajectories of primary producer communities during the last ~100 years in WCA-2A, and these changes are linked with changes in water quality and quantity over that period (summarized in E. Suppl. Mat. Figure 1). Each dated core contained a distinct marl layer, as inferred from a substantial decrease in LOI percentages and a shift to periphyton dominance of the primary producer community with abundant scytonemin, canthaxanthin and myxoxanthophyll concentrations. For the northern-most core, EV-3, LOI changes and photosynthetic pigment profiles show the marl periphyton community beginning ca. 1925. This periphyton community shifted to a Typha dominated community around 1960, which has persisted to the present. Canal construction began in 1917 around WCA-2A and reduced water levels until the impoundment began in the early 1950s (Light and Dineen 1994; Cohen et al., 1999; Willard et al. 2001). The establishment of the calcareous periphyton in the northern most area of WCA-2A occurred before intensive agriculture and impoundment started in earnest, suggesting that canal-derived calcium inputs place a substantial role in initiating this community shift. Lower water levels and reduced hydroperiods during this period of early drainage also could have increased Ca-saturation conditions for biological CaCO3 precipitation (Browder et al. 1994). For the middle core, EV-2, the marl layer established slightly before or around 1950 and was replaced by Typha dominance ca. 1995. For the southern-most core, EV-1, the periphyton community, inferred from marl deposition, also initiated around or slightly before 1950 and continued until present. For both of these sites, the timing of the establishment of the periphyton community, inferred from marl deposition and increased algal pigments, is limited by the coarse resolution of the age models. However, that the marl layers more clearly pre-date 1950, and the improved age resolution in the EV-3 core indicating similar pre-impoundment changes, effects of early drainage and mineral inputs associated with canals likely played a role in initiating the marl-depositing, periphyton communities. However, increases in algal pigment concentrations after 1950 would further suggest the effect of impoundment (1950s) and increasing agricultural inputs continued to alter primary producer community structure in the Everglades. Previous paleoecological work using siliceous macrofossils, pollen and petrographic analyses supports the eutrophication of WCA-2A following 1950 and the impoundment (Cohen et al. 1999; Slate and Stevenson 2000; Willard et al. 2001).
Collecting cores in wetland environments that accurately represent historic change can be difficult due to changing hydroperiods, drying events, fire impacts and breaks in deposition. Prior to core collection, historic sloughs were identified using aerial photographs in an effort to collect cores from more depositional environments. The three cores collected for this study contained sediments deposited prior to major human impacts to the Everglades (pre-1900). Each core contained a distinct low-LOI (marl) layer documenting periods where the primary producer community was majorly periphytic agal mats. Also, non-marl sediments deposited above the marl layer in EV-2 and EV-3 contained distinct Typha fragments indicating Typha dominance of the photosynthetic community. In addition to lithologic changes, the cores also showed gradual decreases in excess 210Pb suggesting a continuous depositional environment, and deposition of excess 210Pb did not appear to be affected by either primary producer community or changes in lithology in each core. As a result, the sediment cores reported here present a viable record of ecological change throughout the period of intense human impacts (≈100 years).
Photosynthetic pigments and primary producer community structure
Photosynthetic pigment profiles and other proxy data support previous research suggesting the Everglades were oligotrophic and phosphorus limited prior to intense human impacts (Noe et al. 2001). For all pigments except for scytonemin, alloxanthin and lutein + zeaxanthin in EV-3, pigment concentrations below the marl layers were extremely low. Because TOC/TN values indicate substantial inputs from macrophyte communities existed in WCA-2A during the pre-modern period (Cohen et al. 1999), low pigment concentrations likely signify reduced algal primary producer abundance, although low pigment preservation (differentiated by site or pigment type) introduces some uncertainty. These sediments are also characterized by lower sedimentary phosphorus concentrations and high LOI and TOC percentages. It is inferred that these core sections represent historic wet prairies containing sawgrass, stonewarts, spikerush and other macrophytes.
Although these data support the existence of pre-marl oligotrophic sloughs, pigment profiles and other proxies suggest that algal communities co-existed in the northern areas of WCA-2A prior to marl development. EV-3 shows a period of significant pigment deposition occurring below the 210Pb-dated portion of the core and included pigments indicative of a diverse primary producer community containing periphyton, macrophytes, cyanobacteria, diatoms and cryptophytes. Scytonemin blocks ultraviolet light and occurs primarily in cyanobacteria (Leavitt et al. 1997). In the Everglades, scytonemin occurs in association with the marl periphyton and other periphytic communities (McCormick and O’Dell 1996). It is inferred from scytonemin occurrence that the northern area of WCA-2A contained a substantial periphyton community before 1900. Based on LOI, pigment and TP profiles, this pre-impoundment periphyton community was not associated with marl. EV-2 contained very low concentrations of pigments prior to impoundment, but the presence of various types of pigments still suggests a diverse mixture of algae including benthic cyanobacteria (canthaxanthin), cyanobacteria + green algae (zeaxanthin + lutein) and cryptophytes (alloxanthin). Cryptophytes are generally planktonic and denote the existence of algal species that were not periphytic (Reynolds 2006). TOC/TN values also suggest an increasing contribution of algal material to the primary producer community from the pre-impoundment period until the present. Prior to 1950 and the establishment of the marl layer, EV-1 experienced significant deposition of pheophytins a and b. These degradation products of chlorophyll a and b suggest conditions that would promote pigment degradation such as increased temperature, light and oxygen. These conditions would occur during dry periods, which occurred in WCA-2A prior to impoundment (Willard et al. 2001). Nevertheless, all pigments were detected during this period, however, at very low concentrations. TOC/TN ratios below the marl for EV-1 were higher than ratios deposited during the marl deposition phase. These values suggest macrophyte dominance with algal occurrence prior to impoundment.
For all three cores analyzed in this study, the first change in community structure was the development of the periphyton algal mats. These algal populations utilize both organic and inorganic forms of phosphorus in the water column (Scinto and Reddy 2003). Periphyton communities can also coprecipitate phosphorus with calcium carbonate (Gleason and Spackman 1974; Browder et al. 1994). As a result, the periphyton mats that developed in the Everglades served as a phosphorus immobilization and storage mechanism to transport phosphorus inputs out of the water column and into the sediments. The spatial differences in the development of the marl periphyton layers suggest that the rate of water column delivery of mineral nutrients governs marl periphyton development. The initial development of marl periphyton communities around 1920 at the EV-3 station and around 1950 at EV-2 and EV-1 was likely a response to changes in inputs of canal-derived minerals (e.g., calcium) and phosphorus. While water column phosphorus gradients in WCA-2A are such that concentrations are greater (>150 μg/l) in the north and lower toward the middle and south (<10 μg/l at 8 km into WCA-2A; McCormick et al. 1996), it is well established that low level P enrichment will cause calcareous cyanobacterial mats to disintegrate (McCormick and O’Dell 1996; Gaiser et al. 2004). Fossil diatoms assemblages from cores in northern WCA-2A indicate moderately eutrophic conditions apparent by ~1940 to 1950 (Slate and Stevenson 2000), which may indicate that water TP was starting to adversely impact the marl periphyton communities already established there. Diatom indicators also suggest that by ~1950 water pH was increasing (Slate and Stevenson 2000), which is consistent with the development of marl periphyton at the interior sites (EV-1 and EV-2) outside of the high P-enrichment zone. Overall, these spatial differences in community changes suggest that both calcium-carbonate and phosphorus delivery play a significant role in the rate of development in periphyton communities. This establishment of marl/periphyton dominance shows spatial and temporal differences demonstrating the heterogeneous nature of the Everglades ecosystem (Wu et al. 2006).
The marl periphyton layer in each core contained the highest pigment concentrations with few exceptions. The marl community at the northern-most station (EV-3) developed earlier than the other two sites. Based on photosynthetic pigment profiles and LOI values, the marl periphyton community existed from 1920 until 1950 and consisted of colonial and filamentous cyanobacteria and diatoms. Most cyanobacterial pigments measured (myxoxanthophyll, canthaxanthin and scytonemin) were highest in concentration during this period. This marl periphyton community abruptly decreased in the mid 1950s and was replaced by a Typha-dominated macrophyte community. Previous research on algal mats in the Everglades noted a change in species concentration from cyanobacteria and diatoms to green algae when phosphorus was added to the community (McCormick and O’Dell 1996; Gaiser et al. 2005). Similar occurrences happened in this study as a long-term trend in periphyton communities. Pigment concentrations corresponding to the marl layer in EV-2 increased around 1950 and achieved maximum concentrations within 5–10 years. In addition, some pigments demonstrated a transition in pigment content in the top 8 cm of EV-2 denoting the establishment of Typha in this area. Myxoxanthophyll, alloxanthin, canthaxanthin, pheophytin b, and scytonemin concentrations are higher in the marl section. Aside from alloxanthin which represents planktonic cryptophyes, the other pigments indicate periphyton mats that are composed of cyanobacteria and green algae. The top samples of the core demonstrate a different primary producer community represented by diatoxanthin, zeaxanthin + lutein, chlorophyll a, chlorophyll b and beta-carotene, which are indicative of both macrophytes and algal primary producers. This community supports current primary producer communities consisting of Typha, other macrophytes and attached epiphytes at this station. In EV-1 pigment concentrations except pheophytins began to increase around 1950 and reached maximum concentrations around 1980, which continued to the top of the core. This increasing trend following impoundment represents a gradual establishment of the marl community for 30 years.
Paleolimnological proxy data shows that the Typha-dominated communities at station EV-3 developed ca. 1965. This inference is based on a decrease in photosynthetic pigment concentrations indicative of marl periphytin communities, an increase in LOI and TOC percentages, TOC/TN values, the occurrence of Typha fragments in the sediments and current day existence of Typha communities. The timing of Typha dominance at EV-3 by ~1965 is in close agreement with the two other paleoecological studies showing initiation of Typha (at nearby sites) by ~1960 (Cohen et al. 1999; Willard et al. 2001). The change to a eutrophic Typha community is also consistent with increasing concentrations of eutrophic diatom indicators from ~1960 to 1970 (Slate and Stevenson 2000). Similar increases in LOI and TOC from the EV-2 core indicate that emergent macrophytes became established at site by ~1990, consistent with aerial surveys of cattail expansion (Urban et al. 1993; Davis 1994), and likely a mix of Typha and white water lily which presently dominate this site (Hagerthey et al. 2008; Rutchey et al. 2008). Although Typha dominance generally overtakes marl periphyton communities, the sections of cores EV-2 and EV-3 suggesting Typha dominance also contain photosynthetic pigments indicating algal occurrence (lutein + zeaxanthin, alloxanthin and diatoxanthin). These pigments could indicate epiphytic algae on the Typha or algae taking advantage of increased phosphorus concentrations and small open water areas. Nevertheless, these data suggest that a substantial algal community does exist in the midst of Typha dominance.
Phosphorus and hydroperiod as drivers of primary producer community structure
The spatial and temporal differences in periphyton (marl-inferred) and Typha development between the cores support previous understanding of water quality and hydrological relationships in WCA-2A. The water-column delivery of mineral nutrients to WCA-2A not only decreases southward but also forms a gradient to the rate of change in primary producer community structure. Based on known events in the management of WCA-2A, changes in community structure are also correlated to hydrological periods. The establishment of the marl periphyton community at stations EV-1 and EV-2 began possibly prior to 1950, indicating an effect of early drainage and mineral inputs from canals, although the age resolution precludes a definitive interpretation. Continued increases in algal pigments after 1950 signal the beginning of impoundment, canal-deepening and increases in agricultural nutrients into WCA-2A (E. Suppl. Mat. Figure 1). The increases in calcium carbonate inputs likely support the establishment of the marl periphyton community, while later increases in phosphorus delivery to WCA-2A through the S-10 structures (starting in earnest in 1961) started the phosphorus-driven feedbacks rapidly leading to Typha dominance from EV-3 and eventually at EV-2. As stated above, phosphorus would continue to be transferred into the sediment by periphyton until concentrations were favorable for Typha communities. The northern most station, (EV-3) clearly developed a marl-forming, periphyton community prior to the impoundment period and intensive agricultural inputs, which supports the importance of calcium carbonate and phosphorus as the primary drivers of primary producer community structure. These inferences support previous understanding of primary producers in WCA2A and show that this study serves as a long-term ecological experiment confirming experimental research.
The inferences based on the three sediment cores collected from WCA-2A and analyzed here demonstrate the ability of paleolimnological techniques to resolve ecosystem changes throughout the past ~100 years. Photosynthetic pigments, LOI, TP, and TOC/TN indicated the important role of mineral inputs, including phosphorus, and hydroperiod in determining the primary producer community. The development of the marl periphyton community was supported by the gradient of water-column mineral inputs and phosphorus both historically and spatially. The periphyton community provided a mechanism to transfer phosphorus from the water column to the sediment, which promoted Typha communities when sediment TP reached 600–700 mg P/kg. A marl-depositing periphyton community developed around 1920 in the northern areas but probably later in the middle and southern areas of WCA-2A, just prior to impoundment in the 1950s. As a result, it is inferred from this study that both calcium carbonate- and phosphorus-driven processes underlie the primary producer community structure in WCA-2A. Hydroperiod may have worked in concert with water chemistry changes by altering solute concentrations supporting periphyton communities. Therefore, future changes to the periphyton and Typha communities will be contingent on calcium carbonate and phosphorus inputs into WCA and the rate of processes known to remove water column phosphorus and deposit it as sedimentary phosphorus, specifically the periphyton mats.
This research was funded by the South Florida Water Management District. Phosphorus was analyzed at the University of Florida Land Use and Environmental Change Institute (LUECI) with the help of William Kenney. Organic carbon and nitrogen were analyzed at Florida State University, National High Magnetic Field Laboratory by Drs. Xu and Wang. Christian Sanders assisted with core collection, and Biliana Ivanova assisted with core collection, core sectioning, sample preparations and 210Pb analysis. This material is also based upon work supported by the National Science Foundation-funded FCE-LTER program under Grant No. DEB-9910514.