Project Coast

The following abstract from the Final Report on Project COAST 1997 - 2005 (Thomas K. Frazer et al. August 2006) is included below for the excellent science, clarity and rationale that it portrays.

Introduction

Compelling evidence links agricultural practices and urban development within coastal watersheds to increased nutrient concentrations in several springs and rivers along Florida's Springs Coast (Jones et al. 1997, Southwest Florida Water Management District 2001). Recent investigations indicate that these riverine systems do not assimilate the full loads of some nutrients, in particular nitrate, with surplus nutrients discharged into nearshore waters of the Gulf of Mexico (Frazer et al. 1998, Frazer et al. 2001a). Thus, the potential for anthropogenic nutrient enrichment and resulting increases in levels of organic matter, or eutrophication, represents a legitimate concern for the nearshore coastal waters along the Springs Coast.

Nutrient enrichment and eutrophication often lead to detrimental effects in coastal systems like those along the Springs Coast (Duarte 1995, Valiela et al. 1997). The vast beds of seagrass that occupy the region are particularly vulnerable (Frazer and Hale 2001, Mattson 2000). Seagrass beds are essential to the health and ecological integrity of Florida's estuarine and nearshore coastal systems because they provide refuge and a habitat for foraging to ecologically and economically important animals, such as scallops, shrimps, blue crabs, myriad fishes, manatees and sea turtles (Killam et al. 1992).

Nutrients, like nitrogen and phosphorus, support the growth of all photoautotrophs, including seagrasses and other benthic macrophytes. Phytoplankton, epiphytic microalgae and epiphytic macroalgae generally take up nutrients more efficiently, especially in nutrient enriched environments (Duarte 1995). For example, seagrasses, phytoplankton and epiphytic microalgae assimilate nutrients from the water column, but phytoplankton and epiphytes outcompete seagrasses, in part due to their rapid uptake, but also because they are located in the water column or on a host plant, which improves contact with water and the nutrients it contains (Williams and Ruckelhaus 1993). In addition to using nutrients more efficiently, both epiphytic microalgae and phytoplankton require less light than slow growing, benthic macrophytes, like seagrasses (Duarte 1995). Numerous studies from around the world indicate that increased nutrient loading to estuarine systems leads to progressive replacement of seagrasses with blooms of fast-growing macroalgae and phytoplankton (Duarte 1995, Valiela et al. 1997). Moreover, available information suggests that damaged seagrass beds require years to centuries to recover (Duarte 1995).

Florida's coastal systems, including Apalachee Bay, Tampa Bay and Florida Bay, have suffered substantial losses of seagrass, with declines often attributed to changes driven by increased nutrient loads (Hale et al. 2004 and references therein). Along Florida's Gulf coast, Hale et al. (2004) reported large-scales shifts in the depth distribution of turtle grass (Thalassia testudinum) and shoal grass (Halodule wrightii), as well as areas of seagrass loss near riverine discharges. They suggested that increased nutrient loading to coastal rivers could underlie these changes.

The potential negative consequences from increased nutrient delivery to estuarine and coastal waters along Florida's Springs Coast are recognized. In response, broad-scale sampling of water quality began in 1997 (Project COAST). The planned, long-term baseline will enable water resource managers to assess changes in nutrient concentrations and eutrophication, with a focus on persistent declines in water quality that might lead to deterioration of seagrasses. Consistent with this objective, we provide updated results that warrant attention because they suggest eutrophication that ultimately may affect seagrasses. Chlorophyll concentrations serve as a proxy for phytoplankton biomass, which is a measure of eutrophication. Along with light availability, it represents one of the two primary determinants of seagrass distribution (Duarte 1991).

Key changes and trends

The effects of the 1998 El Niño event were clearly evident and served to illustrate two important points: (1) water quality parameters vary in estuarine systems and (2) only a sustained, long-term, sampling effort will discern natural variation from human-induced changes in water quality. Broad-scale increases in total nitrogen, total phosphorus and chlorophyll were measured during the El Niño of 1998 (Figures 2-4 Not included). Light attenuation was not measured until the spring of 1999, but Secchi depth readings indicated that water clarity was markedly reduced for the better part of the year (Figure 5 Not included). The system was resilient, with routine water quality parameters returning to "typical" values over a 6-8 month period.

In contrast to this short-term response, chlorophyll concentrations and extinction coefficients along the Springs Coast and in the Withlacoochee system increased consistently from 1999 to 2005 (Figures 6-9 Not included). Increased nutrient loads represented a likely cause of this trend. In particular, the positive empirical relationship between chlorophyll and total phosphorus and the less consistent relationship between chlorophyll and total nitrogen pointed to increased phosphorus as a likely driver (Figures 10 and 11 Not Included). Mean total phosphorus concentrations increased in the Withlacoochee system, but they did not increase along the Springs Coast (Figures 12 and 13 Not included). Total concentrations measured combined changes in several pools of nitrogen and phosphorus rather than changes in bioavailable nutrients. Increases in bioavailable forms may have had unusually large effects on chlorophyll. For example, a 1 g L-1 addition of phosphate may have caused a 1 g L-1 increase in chlorophyll. Such an addition would represent a relatively small change in the pool of total phosphorus, which typically exceeded 10 g L-1 along the Springs Coast. In addition, dilution may have had an effect, because salinity, which represented a conservative indicator of dilution, decreased consistently along the Springs Coast and in the Withlacoochee system (Figures 14 and 15 Not included).

In addition to changes in bioavailable phosphorus or nitrogen, changes in the phytoplankton community may have caused increased chlorophyll concentrations. Available data did not allow an evaluation of this hypothesis.
Regardless of the mechanism(s) that led to increased chlorophyll concentrations and extinction coefficients, the broad-scale trends suggested increased organic matter or eutrophication and the potential for shading of seagrasses. Continued baseline sampling is required to differentiate short-term responses to variations in climate and weather from long-term trends related to human activities. In large part due to climatic variations, answers to critical questions about estuarine and coastal water quality will only become clear over decadal time scales (Enfield et al. 2001, Sutton and Hodson 2005).

Concluding remarks

The potential impacts of increased chlorophyll and decreased light availability on seagrasses and their associated fauna are a primary concern for water resource managers. Over time, high chlorophyll concentrations and resultant shading can lead to loss of seagrasses (Duarte 1991). Because seagrasses represent a valuable habitat and they integrate water quality over time, data on seagrass distribution and abundance would provide a complementary means of assessing important trends in water quality. The absence of such data can compromise inferences about the effects of deteriorating water quality at the ecosystem level and management actions to address these issues.

Natural variability driven by long-term climate and short-term weather provide the context for interpreting our water quality data. This report informs water resource managers of emerging patterns and potentially important issues that can affect decisions about auxiliary diagnostic work and management actions. Clearly, there is a need for a quantitative understanding of nutrient loads within the region. The robust empirical relationship between chlorophyll and total phosphorus concentrations and results of nutrient addition assays point to phosphorus as the limiting nutrient across much of the region (Frazer et al. 1998; Frazer et al. 2001b; Frazer et al. 2002; Hoyer et al. 2002; Frazer et al. 2003; Frazer et al. 2004). However, estimates of all nutrient loads remain essential for quantifying responses and attributing causal links at the appropriate spatiotemporal scales. Data on nutrient loads and models of ecosystem responses will never obviate the need for sustained monitoring of water quality in a program designed to detect unforeseen changes and determine ongoing success of management actions.