Coastal River Systems
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Chassahowitzka River
River Description
The Chassahowitzka River, located in southwest Citrus County, is a spring-fed system within the Chassahowitzka Coastal Strip with elevations below 3 m. This area is dominated by flatwoods and swamps that transition into an extensive marsh complex along the Gulf of Mexico. Of the three rivers in the University of Florida study, the Chassahowitzka River is the least developed. A small residential community and fish camp is present near the headspring and there are also a series of man-made canals immediately adjacent to the area. Development along the river is limited to approximately a dozen homes in the lower river.
The river runs west approximately 4 km from the main spring boil to the beginning of the associated coastal marsh complex and then another 4 km to the Gulf of Mexico. The upper portion of the Chassahowitzka River is narrower, approximately mid-way down-stream the river rapidly widens to a maximum width of 175 m. Above the marsh complex, mid-stream channel depth is on average ca. 1.2 m (Frazer et al. 2001a).
The majority of stream discharge emanates from a main spring boil, however, several smaller spring runs in the upper river contribute additional flow (e.g., Chassahowitzka #1, Crab, Baird, and Potter creeks; see Jones et al. 1997 and references therein). Tidal cycles influence both spring discharge and flow within the river (Yobbi 1992). The average discharge between 1998 and 2000, just below Crab Creek, was reported by Frazer et al. (2001a) to be 4.0 m3 s-1. This value is comparable to the long-term average reported by Jones et al. (1997) for the main spring between 1930 and 1972, i.e. 3.9 m3 s-1, but less than the combined discharge of the spring complex that includes the aforementioned smaller spring runs.
In general, the substrate within the river is dominated by sand or sand/mud mixtures, except near the fringes of the shoreline where mud is more prevalent. Small patches of exposed limestone occur sporadically throughout the river. Submersed aquatic vegetation is nearly ubiquitous, though density tends to decline with distance downstream (see Yobbi 1992; Frazer et al. 2001a), due, in large part, to increased salinity (see Hoyer et al. 2004). Filamentous macrolagae was reported to dominate the SAV community between 1998 and 2000, and was particularly abundant in the upper 2 km of the river (Frazer et al. 2001a). Common macrophytes observed during this time period included Vallisneria americana, Potamogeton pectinatus, Najas guadalupensis, Myriophyllum spicatum, andHydrilla verticillata.
An extensive marsh system occurs at the mouth of the river and upper estuary. Seaward of the marsh, the water is generally shallow and interspersed with numerous small islands. Some patchy seagrass exists in the estuary seaward of the marsh complex, but macroalgae are more prevalent (Dixon and Estevez 1997). Both attached macroalgae, e.g. Caulerpa sp., and unattached (drift) forms are frequently observed in this estuary.
Temperature
Groundwater emanating as spring discharge is the primary water source for the Chassahowitzka river. Water temperatures, as a consequence, are fairly stable in the uppermost portions of the river. In fact, mean recorded water temperatures at the upper most river transects varied by less than 1°C across the two sampling intervals (N=66) with a range of 23.2 to 23.9 °C. Variations in surface water temperature become more pronounced downstream as a result of atmospheric warming or cooling, and are particularly variable in the mixing zones seaward of the marsh complex.
The Chassahowitzka River is sufficiently wide that terrestrial canopy coverage has little influence on incoming solar radiation (see Frazer et al. 2001a). Within the two project periods, mean water temperatures were fairly uniform between transects 1 and 15 . Variation around the calculated mean values increased with distance downstream and was most pronounced in the lower river. Overall, water temperatures were significantly lower (ANOVA, df = 19, F = 58.38, p < 0.001) between 2003 and 2005 than between 1998 and 2000, reflecting seasonal variations.
Stream Velocity
The rate of water movement through aquatic systems can have a profound effect on their physical and biological structure. With regard to the biological structure of rivers and streams, it is generally considered that water velocity is the most important environmental factor affecting the abundance and distribution of plants and animals (see Allan 1995). With respect to our interest here, it should be noted that stream velocity determines not only the water residence time, but also the substrate characteristics which are also primary determinants of the abundance and distribution of aquatic plants and algae (Butcher 1933; Canfield and Hoyer 1988b).
The Chassahowitzka river is a low gradient coastal stream with slow to moderate stream velocities (see Hoyer et al. 2004). It is tidally influenced and flow reversals are common in the lower portions, particularly in the marsh complexes. Reverse flows were evident sometimes in the upstream areas. The directional changes in flow are important as that they can affect increases in water residency time, particularly in the lower river and marsh transition zones, which can lead to the accumulation of phytoplankton biomass. In addition, tidally induced flow reversals introduce relatively high salinity water into the rivers that, in turn, can affect the species composition of plants and algae and influence their broad patterns of abundance and distribution (Hoyer et al. 2004). In many instances, however, it is not the direction of flow that is important, but the absolute velocity of flow. High water velocity, whether occurring as a consequence of water moving downstream or an incoming tide, has the capacity to influence the substrate characteristics and also the colonization capabilities of certain plants and algae. Low flow rates, coupled with increased nutrient concentrations, may promote the accumulation of periphyton associated with submersed aquatic vegetation (including native macrophytes) in the rivers with negative consequences. All of the scenarios above are relevant to this investigation and are discussed in later sections of the text where appropriate. The purpose of this section is intended primarily to describe the flow characteristics observed in the study system.
In general, flow rates averaged across all sampling periods were positive, irrespective of project period. Within the marsh transition zones of the river, however, reverse flows due to incoming tides during sampling and resulted in average negative values at several transect locations on several occasions . Highest mean flows (greater than 0.3 m s-1) were most frequently measured in the narrowest regions. Lower mean flows (< 0.10 m s-1) were generally observed in the Chassahowitzka and Homosassa systems. Flow rates were lower during the 1998 to 2000 project period when compared to the 2003 to 2005 project period. Flow rates were determined only at upper Transects; no data were collected from open water stations seaward of the marsh complex.
Within the Chassahowitzka River mean flow rates at the uppermost transect, were 0.11 and 0.12 m s-1, respectively, for the 1998-2000 and 2003-2005 project periods . There was a significant increase in flow between project periods (ANOVA, df = 19, F = 4.86, P = 0.04). Interestingly, during one sampling event (May 2000) reverse flow was measured at the upper most transect (and throughout the river) indicative of the extreme influence of tidal stage on the flow characteristics of this river during drought conditions (see Frazer et al. 2001a). Flow rates in the marsh and estuary exhibited a wide range of values (less than -0.7 to greater than 0.5 m s-1) and reflected the fact that sampling was carried out during both ebb and flood tides.
The lower flow rates during the 1998-2000 project period can be attributed to reduced rainfall in the region, lower recharge and subsequent declines in spring discharge.
Water Depth
Being tidally influenced, some variation in depth measures are, therefore, to be expected as a consequence of sampling during different tidal stages, especially in marsh transition zones and open water sites. With this caveat in mind, there were no significant differences in mean depth for the Chassahowitzka river between the 1998-2000 and 2003-2005 project periods.
In the Chassahowitzka River, shallowest mean depths (1 m or less) were generally measured between Transects 5 to 7. Water depth generally increased from Transect 7 to Site 16 and a maximum value of 4.8 m was recorded at the latter site. Seaward of the marsh complex, depths decreased and mean values were generally between 2 and 3 m.
Stream Discharge
Stream discharge is of primary interest as it affects the supply, i.e. loading rates, of nutrients to the rivers and adjacent estuarine and coastal waters. The tidal nature of the systems investigated complicates discharge calculations, especially in the lower portions of the rivers. Thus, only those measures made at the most upstream transects were used in subsequent calculations of nutrient loading rates (see discussion of Chemical Parameters below).
For consistency in data presentation we provide calculated mean discharge measures for those transects above the marsh transition zones, i.e. transects 1 through 10, in each river system. In almost all cases, the calculated measures of discharge (based on stream velocity, depth and width; were greater in the 2003-2005 project period than in the 1998-2000 project period. This is not surprising given the concordant measures of stream velocity and similarities in stream depth between the two project periods. The increased discharge rates (and also stream velocities) in 2003-2005 can likely be attributed to increased rainfall and subsequent recharge of the aquifer system that supplies water to the springs that, in turn, serve as the origin of stream flow.
The rainfall data presented are for the broader Springs Coast region which includes the watersheds of the Weeki Wachee, Chassahowitzka and Homosassa Rivers (SWFWMD unpublished data). The time period of record (1915-2005) represents 91 years. During this time interval, the annual average rainfall was 54.97 inches (std. dev. 8.54 and 95% CI of 53.19 to 56.75 inches). It is clear from these data that the three year period between 1998 and 2000 was one of low precipitation. In fact, the rainfall amount in 2000 was the lowest on record. In contrast, the 2003-2005 time period was relatively wet. In fact, rainfall in each of those years was greater than the long-term average of 54.97 inches per year.
Substrate Type
The bottom sediments in river was categorized in conjunction with the vegetation sampling effort carried out during summer months during both project periods (see Vegetation Sampling below). In general, each river was dominated by sand and mud or a combination of the two substrate types as is characteristic of low-gradient systems (Allan 1995). These substrate types are considered, in most cases, favorable for the colonization and growth of rooted plants, and historically, the Chassahowitzka river was presumed to be dominated by native macrophyte species such as Vallisneria americana and Sagitarria kurziana.
Qualitatively, the frequency of observed substrate types was similar during both project periods, i.e. 1998-2000 and 2003-2005. The minor differences observed among years can likely be attributed to sampling variability and appear to be little reason for concern. With that said, however, there has been no attempt to more rigorously assess the sediments with respect to mineral composition, organic matter content or other chemical characteristics that may affect the ecology of these systems. In fact, increased organic matter content as a consequence of increased primary production and subsequent decay of plant and or algal material should be a concern in any system that has been subjected to increased nutrient delivery. With regard to the Chassahowitzka river, slow to moderate flows coupled with the accumulation of fine grain organic material and other organic detritus could lead to increased biological oxygen demand altering sedimentary chemical fluxes that, in turn, mediate biological and ecological processes. For example, an enhancement of sulfate reduction rates in vegetated sediments (Holmer and Nielsen 1997; Holmer et al. 2003) as a consequence of increased organic carbon input (and associated increased biological oxygen demand) may induce a negative feedback on the plants themselves, as sulfate reduction results in the release of sulfide, which can be toxic (e.g., Terrados et al. 1999). Increases in the spatial extent or frequency of occurrence of anoxia and/or hypoxia at the sediment/water interface can also affect the abundance and distribution of faunal organisms and may result indirectly in altered predator/prey dynamics of key species (see, for example, Eby et al. 2005, Eggleston et al. 2005). It is worth noting that on several occasions during our sampling effort, we observed bottom oxygen concentrations at or near 0 mg/L in the middle and lower portions of the Chassahowitzka river (Frazer et al. unpublished data). Such observations, in combination with the general decline in vegetation in these rivers (as reported elsewhere), are consistent with a eutrophication scenario and should compel water resource managers to investigate further the sediment dynamics in this system.
Light Attenuation
It has already been established that light availability is not impacted substantially by terrestrial canopy coverage in Chassahowitzka river, and it is unlikely that terrestrial canopy coverage is the primary determinant of submersed plant abundance or distribution. Other factors, however, do have the potential to affect the light environment in these and other aquatic systems. Suspended solids and dissolved organic material in the water, for example, are the primary factors affecting light transmittance (Kirk 1994). Suspended solids include such constituents as algal cells (Canfield and Hodgson 1983), non-volatile suspended solids (Canfield and Bachmann 1981, Hoyer and Jones 1983) and detrital material (Buiteveld 1995). Because the Chassahowitzka river is primarily spring-fed, the terrestrial sediment load is minimal. Suspended organic material in the river is generally comprised of algal cells (phytoplankton and/or dislodged periphyton) and/or plant/algal detritus. Dissolved organic materials include plant and algal exudates that originate in the rivers and also humic and fulvic acids which are derived from decaying terrestrial vegetation. These latter substances contribute color to the water and originate in adjacent lowland forests and swamps as well as in the marsh complexes. Increased precipitation and subsequent discharge of recently derived groundwater from the surficial aquifer and overland flow are expected to affect an increase in colored dissolved organic material and associated color values. Similarly, increases in discharge and concomitant increases in stream velocities might be expected to increase the occurrence of dislodged periphyton in the water. In either case, light attenuation will be increased (reduced light).
In the Chassahowitzka River, mean light attenuation coefficients (Kd m-1; averaged across years during each of the two project periods) were always less than 2.0 Kd m-1). As in the Weeki Wachee River, the lowest mean light attenuation coefficients (< 1.25 Kd m-1) were typically observed in the upper section of the river in close proximity to the main spring inputs. Maximum mean light attenuation coefficients (> 1.75 Kd m-1) were typically observed in the lower river and marsh transition zone. Light attenuation coefficients were significantly greater in 2003-2005 than they were during the initial project period (ANOVA, df = 19, F = 32.86, P < 0.0001).
Frazer et al. (2001a) reported that the euphotic depth was often shallower than the bottom depth in the lower portion of the Chassahowitzka river. This finding implied that the light environment was likely to limit the downstream extent of submersed aquatic vegetation in these two systems. The increase in light attenuation coefficients during 2003-2005, suggests that the downstream distributional range of submersed aquatic vegetation may have been further restricted than in 1998-2000 and may explain, in part, the general reduction in aquatic macrophytes in this system during the most recent sampling interval.
Chemical Parameters
Nitrate
As indicated previously, nitrate is a nutrient of particular concern. Jones et al. (1997) were, to our knowledge, the first to document increasing nitrate concentrations in each of the spring systems investigated here. These investigators suggested that further increases in nitrate concentrations were likely to occur as a consequence of the lag-time associated with groundwater movement, and the fact that nitrate that had been supplied to the region for many years was now entrained in the aquifer. The data reported herein indicate that this is, indeed, the case.
Nitrate concentrations have increased significantly in all three spring-fed river systems. Most notable has been the change in the Weeki Wachee River and we draw the readers' attention to the following observations. The mean nitrate concentration at the uppermost sampling transect in the Weeki Wachee River between 1998 and 2000 was 524 µgN L-1. Between 2003 and 2005, the mean nitrate concentration at this same transect was 781 µg L-1 . This more recent value represents an approximate 50% increase in nitrate concentration relative to that reported for the earlier time period. Similar comparisons indicate a 20% increase in the mean nitrate concentration at the uppermost sampling transect in the Chassahowitzka River and a 6% increase in the Homosassa River. The increases in nitrate concentrations coupled with concomitant increases in flow rates and calculated discharge in each of the three rivers has resulted in substantially greater nitrate loads. In fact, calculated nitrate loading rates in the headwater regions of the Weeki Wachee, Chassahowitzka and Homosassa rivers have increased by 76%, 43% and 56%, respectively, since the 1998-2000 sampling period. These findings are reason for concern, given the increased potential for eutrophication of downstream receiving waters, and warrant the close attention of water resource managers. A closer inspection of the nitrate data collected during 1998-2000 and 2003-2005 are provided below.
Within the Chassahowitzka River, the mean nitrate concentration at the uppermost transect was 401 µgN L-1 during the 1998-2000 sampling period and 483 µgN L-1 during the 2003-2005 sampling period. Nitrate concentrations declined steadily with distance downstream during both the first and second project periods, but mean values at any given sampling transect above the marsh complex were always greater during the most recent sampling period, i.e. 2003-2005. Mean nitrate concentrations also tended to be greater in the marsh transition zone during 2003-2005 than during the 1998-2000 sampling period. Overall, the increase in nitrate concentrations was highly significant between the two sampling periods (ANOVA, df = 19, F = 67.61, P < 0.0001). Submersed aquatic vegetation (including periphyton associated with macrophytes and macroalgae) undoubtedly contribute to the draw-down of nitrate in this system, but other biogeochemical processes, e.g., denitrification, are likely removal mechanisms as well, particularly in the lower river and marsh transition zone where plants and algae are less abundant and dissolved oxygen concentrations at the sediment/water interface are often depleted (Frazer et al. unpublished data).
Ammonium
In comparison to nitrate, ammonium is a much less abundant chemical species in the rivers investigated here. Concentrations are, in fact, typically more than an order of magnitude less than nitrate concentrations.
Mean ammonium concentrations in the Chassahowitzka River were generally less than 30 µgN L-1 above the marsh complex during both sampling periods. In the lower marsh transition zone and in the open water sites, mean ammonium concentrations tended be higher in 1998-2000 than in 2003-2005. As a consequence, there was a statistically significant difference in this variable between the two project periods (ANOVA, df = 19, F = 43.94, p < 0.001).
Soluble Reactive Phosphorus
Although much attention has been paid during the last decade to the issue of increasing nitrate concentrations in Florida's springs, it has become evident in recent years that this is not the only nutrient of concern. Phosphorus is another. Strong empirical relationships between phosphorus and chlorophyll were established for surface waters in the Springs Coast region in the late 1990s (Frazer et al. 1998), and subsequent experimental work indicated that phosphorus, rather than nitrogen, often limits primary production in both riverine and coastal waters (Frazer et al. 2002). In a related effort, Notestein et al. (2003) determined that additions of phosphate resulted in greater periphyton growth in the Chassahowitzka River than did additions of nitrate.
In comparison to nitrate, data on phosphorus concentrations in Florida's springs or spring-fed rivers are scarce. Thus, it is has not been possible to say definitively whether or not phosphorus concentrations have increased in a like manner. It is not unreasonable, however, to assume that they have given that fertilizers, which have been identified as the primary source of nitrogen in groundwater (Jones et al. 1997), are also generally comprised of phosphorus. The analysis of the data collected between 1998-2000 and 2003-2005, to our knowledge, provided the first opportunity to assess temporal differences in phosphorus concentrations in the Weeki Wachee, Homosassa and Chassahowitzka rivers that may be reflective of longer-term changes in this nutrient. The findings merit the close attention of water resource managers.
Concentrations of soluble reactive phosphorus, a biologically available constituent of the total phosphorus pool, increased significantly in all three river systems between the two project periods. Increases of 19% were documented in the upper regions of the Chassahowitzka river. As a consequence of increased discharge (see discussion above), loading rates calculated at the uppermost sampling transects in the Chassahowitzka river increased by 44% between the 1998-2000 and 2003-2005 sampling periods. A closer inspection of the soluble reactive phosphorus data collected during 1998-2000 and 2003-2005 are provided below.
As previously indicated, soluble reactive phosphorus concentrations within the Chassahowitzka River, were significantly greater during the 2003-2005 sampling period than during the 1998-2000 sampling period (ANOVA, df = 19, F = 11.20, P < 0.01). As for nitrate, mean soluble reactive phosphorus concentrations were always highest at the upper most transect (between 14 and 16 µg P L-1), but declined steadily downstream, presumably due, in large part, to uptake and assimilation by submersed aquatic vegetation (including periphyton associated with macrophytes and macroalgae).
Total Nitrogen
The vast majority of the nitrogen supplied to the rivers investigated here is in the form of nitrate. Total nitrogen concentrations increased also in the Chassahowitzka and Homosassa rivers between the 1998-2000 and 2003-2005 project periods (ANOVA, df = 19, F = 46.76, p < 0.0001 and F = 78.70, p < 0.0001, respectively). Initial declines in total nitrogen were observed in the upper portions of both of these rivers coincident with the draw-down of nitrate. In the Chassahowitzka River, however, there was a notable increase in total nitrogen concentrations between transects 6 and 10 during both project periods, a pattern that can likely be attributed to autochthonous inputs, e.g., vascular plant and algal debris, sloughed periphyton, phytoplankton and/or extracellular materials released by plants and algae as a consequence of routine metabolic activities.
Total Phosphorus
Statistically significant differences in total phosphorus concentrations were observed between the two project periods, i.e. 1998-2000 and 2003-2005, for each of the three rivers investigated. Little variation in total phosphorus concentrations was observed between years at the uppermost sampling transects of any of the three rivers, indicating that differences were due largely to differences downstream.
A comparison of the 1998-2000 and 2003-2005 data also indicated an overall decrease in total phosphorus concentrations for the Chassahowitzka River system (ANOVA, df = 19, F = 9.40, p < 0.01). This decline can be attributed largely to high algal biomass in the lower portion of the river above the marsh complex during a single sampling event that coincided with the aforementioned El Niño event during the 1998-2000 time period .
Salinity
Mean salinities over the course of this investigation were fairly uniform within each of the rivers above their respective marsh transition zones. Mean salinities increased sharply in each of the systems as water flowed through the marsh transition zones and into the estuary as a consequence of mixing with more saline water from the Gulf of Mexico. Overall, salinities were significantly less in 2003-2005 than in 1998-2000 (ANOVA, p < 0.0001 for all three rivers). This finding is consistent with the increase in rainfall and corresponding increases in flow rates and discharge that occurred during the more recent sampling interval, i.e. 2003-2005 (see Results and Discussion above).
Total Alkalinity and pH
The three rivers investigated here are alkaline, highly buffered systems, and are similar in this regard to many of Florida's spring-fed systems (Beck 1965; Canfield and Hoyer 1988a). Although several statistically significant differences in total alkalinity (mg L-1 of CaCO3) and pH were evident between the 1998-2000 and 2003-2005 project periods , we suggest that the actual magnitude of the differences were not sufficiently large to warrant concern from either a biological or ecological perspective. Such differences are, in fact, to be expected as dissolution of carbonate minerals in karst aquifers results in rapid and extensive interactions of surface and ground water. The intricacies of these interactions are not fully understood, but are likely influenced by variations in climate and the hydrologic regimes unique to each of the systems.
Dissolved Oxygen
Low dissolved oxygen concentrations in ground water emanating as spring discharge are not uncommon (see Rosenau et al. 1977). In each of the three rivers investigated, dissolved oxygen concentrations were always lowest at the upper sampling transects in closest proximity to the main spring vents. Dissolved oxygen concentrations, however, increased rapidly with distance downstream, due to atmospheric exchange and oxygen liberation by submersed aquatic vegetation (see Frazer et al. 2001a). The daytime surface water measurements of dissolved oxygen concentration reported herein appear to indicate that each of the three systems is well oxygenated during the day; overall mean dissolved oxygen concentrations always exceeded 6 mg L-1. Overall, dissolved oxygen concentrations increased significantly between the two project periods in the Chassahowitzka River (ANOVA, df = 19, F = 36.68, p < 0.0001).
It is important to recognize that all oxygen measures reported here were made just below the surface during daylight hours. Thus, they are likely higher than similar measures would have been if taken just before dawn. It is also likely that measures near the bottom sediments would have been lower, particularly in the lower portions of the rivers. In fact, we have recently measured dissolved oxygen concentrations near 0 mg L-1 in the lower Chassahowitzka River and lower Homosassa River in areas above the marsh transition zone (Frazer et al. unpublished data). These extremely low values may be due to an accumulation of organic matter and respiration by the associated microbial communities. Given the concerns over nutrient enrichment and potential eutrophication of these rivers, these ancillary observations of anoxic conditions merit further investigation.
Chlorophyll
As indicated by Frazer et al. (2001a), the physical nature of the Weeki Wachee, Chassahowitzka and Homosassa rivers is such that the phytoplankton biomass would not be expected to accumulate in these systems. Stream lengths between the main spring or complex of springs where flow originates and the marsh complex is less than 7 km and calculated water residence times in the upper portions of each of the rivers are the order of hours; a much greater water residency time than would be necessary to support the development of a substantial population of phytoplankton. With that said, however, elevated chlorophyll (a proxy measure for phytoplankton biomass) concentrations (>10 µg L-1) have been previously documented in the lower portions of both the Chassahowitzka and Homosassa rivers and their respective marsh transition zones, where water residency times can presumably be increased as a consequence of tidal forces (see Frazer et al. 2001a).
A comparison of the 1998-2000 and 2003-2005 data also indicated an overall decrease in mean chlorophyll concentrations for the Chassahowitzka River system (ANOVA, df = 19, F = 20.75, p < 0.001).
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