**1.5. Water quality: suspended solids, turbidity, phosphorus, and chlorophyll-***a*

The water quality conditions in many fresh and coastal marine systems have progressively declined as the human population increases last century. This is often in response to increased runoff of nutrients from encroaching agricultural and urban areas due to housing development [77–79].

Over half of the Blackbird Creek Watershed is composed of agricultural lands. These lands are fertilized heavily to accommodate the timely growth of corn, soybeans, and sorghum grass. This fertilization is a concern for marsh habitat and local nekton of commercial and ecological importance. The excess nutrient loading tends to favor the growth of aggressive, invasive, often weedy, plant species which displace native plants. In extreme cases, nitrogen and phosphorous loading cause algae growth so great that the blooms block out sunlight to submerged aquatic vegetation, a crucial habitat for larval and juvenile fish and crabs [78, 79].

Stone [2] discussed that the overabundance of both reactive nutrients and non-reactive constituents can be detrimental to the quality of waterways, while they may not be acutely toxic. Turbidity is a measure of how clear or cloudy a water sample is and, by proxy, a suitable measurement to determine suspended sediments and solids which could settle out onto the riverbeds and banks [80]. Davies-Colley and Smith [81] reported that sediments in suspension can have the potential to smother benthic biota, irritate fish, crab and shellfish gills, and transport adsorbed contaminants. While sediments do have the capability of settling out, there is also a potential for them to remain in suspension and reduce the visual range of sighted organisms to seek out prey and/or members of the opposite gender for reproductive activity. High turbidity reduces the light penetration into the water and lowers the amount of available light availability. This, in turn, reduces the amount of phytoplankton production, possibly impacting higher trophic levels in a bottom-up model [82–85].

De Robertis et al. [83] and Kirk [86] documented the role of phosphorous in receiving waters. In systems like Blackbird Creek and Choptank River, the contribution of phosphorous to surface water is more likely from crop fields instead of household wastewater. Olli et al. [87] discussed that most of the phosphorous is generally in the form of bound reactive phosphate attached to eroded particles that have run off into the surface water not readily available for uptake by plant. On the other hand, Correll [88] and Reddy et al. [89] reported that phosphorous was retained in water and sediments via adsorption, complex formation, chemical precipitation, and biogeochemical reactions or recycled in the system. The compounds may be enzymatically hydrolyzed to form orthophosphate once phosphorous enters the water. This is the only form of phosphorus that can be taken up by algae, plants, and bacteria.

Blackbird Creek and other Mid-Atlantic watersheds influenced by agricultural land-use practices are susceptible to phosphorous enrichment. Many agricultural crops that are grown adjacent to waterways are treated with fertilizer containing high phosphate concentrations. As most of the phosphorus that enter aquatic environment precipitate by interacting with cations, phosphorous in freshwater can be retained in sediments by interactions with cations such as iron (Fe) and aluminum (Al). In seawater environments, deposited phosphorous is in large part returned to the overlying water through remineralization, making it biologically available for the consumption of phytoplankton and have better cycling of phosphorus [90].

**1.5. Water quality: suspended solids, turbidity, phosphorus, and chlorophyll-***a*

ment [77–79].

182 Water Quality

crabs [78, 79].

and bacteria.

The water quality conditions in many fresh and coastal marine systems have progressively declined as the human population increases last century. This is often in response to increased runoff of nutrients from encroaching agricultural and urban areas due to housing develop-

Over half of the Blackbird Creek Watershed is composed of agricultural lands. These lands are fertilized heavily to accommodate the timely growth of corn, soybeans, and sorghum grass. This fertilization is a concern for marsh habitat and local nekton of commercial and ecological importance. The excess nutrient loading tends to favor the growth of aggressive, invasive, often weedy, plant species which displace native plants. In extreme cases, nitrogen and phosphorous loading cause algae growth so great that the blooms block out sunlight to submerged aquatic vegetation, a crucial habitat for larval and juvenile fish and

Stone [2] discussed that the overabundance of both reactive nutrients and non-reactive constituents can be detrimental to the quality of waterways, while they may not be acutely toxic. Turbidity is a measure of how clear or cloudy a water sample is and, by proxy, a suitable measurement to determine suspended sediments and solids which could settle out onto the riverbeds and banks [80]. Davies-Colley and Smith [81] reported that sediments in suspension can have the potential to smother benthic biota, irritate fish, crab and shellfish gills, and transport adsorbed contaminants. While sediments do have the capability of settling out, there is also a potential for them to remain in suspension and reduce the visual range of sighted organisms to seek out prey and/or members of the opposite gender for reproductive activity. High turbidity reduces the light penetration into the water and lowers the amount of available light availability. This, in turn, reduces the amount of phytoplankton production, possibly impact-

De Robertis et al. [83] and Kirk [86] documented the role of phosphorous in receiving waters. In systems like Blackbird Creek and Choptank River, the contribution of phosphorous to surface water is more likely from crop fields instead of household wastewater. Olli et al. [87] discussed that most of the phosphorous is generally in the form of bound reactive phosphate attached to eroded particles that have run off into the surface water not readily available for uptake by plant. On the other hand, Correll [88] and Reddy et al. [89] reported that phosphorous was retained in water and sediments via adsorption, complex formation, chemical precipitation, and biogeochemical reactions or recycled in the system. The compounds may be enzymatically hydrolyzed to form orthophosphate once phosphorous enters the water. This is the only form of phosphorus that can be taken up by algae, plants,

Blackbird Creek and other Mid-Atlantic watersheds influenced by agricultural land-use practices are susceptible to phosphorous enrichment. Many agricultural crops that are grown adjacent to waterways are treated with fertilizer containing high phosphate concentrations. As most of the phosphorus that enter aquatic environment precipitate by interacting with cations, phosphorous in freshwater can be retained in sediments by interactions with cations

ing higher trophic levels in a bottom-up model [82–85].

RMB Environmental Laboratories Inc. [91] reported that chlorophyll-*a* is a green pigment in plants, algae, and phytoplankton that allows photosynthesis. The amount of phytoplankton growth in water bodies depends on various factors such as water temperature, water transparency, predation by zooplankton, and the availability of phosphorus and nitrogen (nutrients). Natural seasonal variations changed phytoplankton concentrations, and drastic changes in the abovementioned parameters due to hurricanes, storms, heat waves, and so on have detrimental effects on overall phytoplankton diversity and biomass [92]. Per RMB Environmental Laboratories Inc. [91], spring time, water is more transparent while there are more nutrients available due to the spring turnover; however, the water temperature is still low and this limits algal growth. As the water temperature increases during the summer, algae grow and its concentrations are higher. More nutrients can get washed into the lake if there are heavy rains that trigger more algal blooms. With increase in algal concentration, water becomes less transparent. Ritchie and Cooper [93] reported that eutrophication can be quantified by the concentration of chlorophylls contained in phytoplankton. According to USEPA (2000), monitoring the concentration of chlorophylls in phytoplankton can be used to evaluate and control eutrophication in water bodies.

According to Fondriest Environmental Inc. [94], the concentration of chlorophyll-*a*, as the green pigment used by phytoplankton in photosynthesis, is a good indicator of the amount of phytoplankton biomass in aquatic environments. Chlorophyll-*a* concentrations can be monitored in several forms consisting of chlorophylls a, b, c, d, and e. Concentrations of those forms determine the trophic state of water bodies, and high concentration most often indicates eutrophication [94].

Chlorophyll-*a* concentration estimated from the blue-to-green ratio of water-leaving radiances is used as a proxy for the phytoplankton biomass—for its distribution, variability, and growth rate studies [95]. This approach has been very successful in providing data for global distributions of algal biomass and in formulating estimates of global ocean primary production [96]. Nevertheless, knowledge of chlorophyll-*a* concentration is not sufficient to properly assess the biogeochemical and geophysical properties of the ocean surface layer [97].

Stumpf et al. [98] published results on monitoring *Karenia brevis* blooms in the Gulf of Mexico using SeaWiFS ocean color imagery, and indicated "the need to detect *Karenia brevis* blooms requires additional capability than simply identifying chlorophyll patterns." The capability of determining algal taxonomic composition can yield important information about overall water quality, eutrophication, and the distribution of HABs. A shift in algal species composition can be an important indicator of changes in water quality and may have serious environmental repercussions [99]. Therefore, the capability of determining this composition, as well as an estimation of algal biomass (chlorophyll-*a* concentration), by remote sensing is needed for the detection and monitoring of HABs. These data, when combined with other data, such as in situ data of nutrients and meteorological data, can potentially create models to forecast the development of blooms and HABs.

Stone [2] reported "as climate changes, it is simply not possible to discuss aquatic ecosystems without a detailed description of current and future trends in temperature, salinity, dissolved oxygen, pH, and the interactive effects of each of these variables on each other. This is also true with respect to change in land use. When land is modified to accommodate human needs, surface runoff, evapotranspiration, stream discharge, and sediment transport are all influenced, which in turn could have implications on the aforementioned parameters. If there is greater runoff from a crop field that was converted from a forested wetland, for example, there may be increased turbidity, reduced water temperature, and lower salinity. The potential ramifications in such a scenario would be monumental." The integrated use of remotely sensed data, geographic information system (GIS), and global positioning system (GPS) has enabled natural resource managers, state and federal governments, and consultants to develop management plans and policies for a variety of natural resource management applications for long-term sustainable options [100].
