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Trophic status related to periphyton and macrophyte distribution and abundance along the North Bosque River in north central Texas.

Article Excerpt
Abstract.-A two-year survey was conducted to estimate periphytic algae and macrophyte diversity and abundance at 11 locations within the North Bosque River watershed. Ash free dry weight by species was used to measure biomass. A total of 30 different species were collected; 24 macrophytes, five periphyton, and one moss. Macrophytes comprised the largest portion of total biomass at most sites, while algae were most frequently collected. The most biomass was contributed by the macrophyte Justicia americana (L.) Vahl, while Cladophora sp. was the dominant algae. The majority of algae were found in riffles, while macrophyte biomass was fairly evenly distributed between pool and riffle habitats. Nutrient concentrations generally showed saturated conditions for periphyton growth indicating that factors other than nutrients, such as light, may be limiting periphyton and possibly macrophyte biomass along the river. In comparison to proposed trophic state indices for periphyton, mesotrophic or eutrophic conditions were indicated at four of 11 sites. In contrast, measurements of instream chlorophyll-[alpha] concentrations of phytoplankton indicated mesotrophic conditions at eight of 11 sites. These differences in tropic status based on periphyton and phytoplankton indicate a need to monitor attached, as well as free-floating algae, when evaluating for water quality impairment in small to intermittent sized creeks and rivers.

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In 1998 the North Bosque River was placed on the Texas Clean Water Act section 303(d) list as an impaired water body due to excessive nutrients and plant growth under narrative water quality standards (TNRCC 1998). Besides narrative standards, in Texas, excessive algal growth may be identified through violation of water quality standards for dissolved oxygen (TCEQ 2003). Concern regarding eutrophication may be defined by comparing nitrogen, phosphorus, or sestonic chlorophyll-[alpha] (CHLA) concentrations to screening levels. These screening levels are based on an 85-percentile ranking of State water quality data for specific water body types (TCEQ 2003). This ranking allows a relative indication of water quality concerns for nutrients and planktonic algae that is statistically derived but has no direct biological meaning.

For the North Bosque River, phosphorus was identified as the nutrient limiting algal growth (Kiesling et al. 2001). Excessive nutrients, particularly phosphorus (P) in freshwater systems, are major factors in causing excessive algal growth (Gibson 1997). To help control algal growth, two total maximum daily loads (TMDLs) were approved with a reduction goal of about 50 percent in soluble reactive phosphorus along the North Bosque River (TNRCC 2001). Routine monitoring, as part of the post-TMDL implementation effort, includes measurement of sestonic CHLA, which primarily represents the phytoplankton or free-floating algae in the water column. Sestonic CHLA concentrations are often used to measure trophic status as an indicator of eutrophication in lakes and reservoirs (Carlson 1977). Trophic boundaries proposed by EPA (2000) for phytoplankton in rivers and streams are 10 [micro]g/L CHLA for the oligotrophic to mesotrophic boundary and 30 [micro]g/L for the mesotrophic to eutrophic boundary.

Although phytoplankton is a good indicator of eutrophication, in most cases phytoplankton measurements alone underestimate algal biomass in wadeable streams. Periphyton, as attached filamentous algae, is often dominant relative to phytoplankton (Allen 1995). Monitoring periphyton biomass is much more time consuming and labor intensive than measuring sestonic algae, so much less research has been done on defining boundaries for trophic classification of periphytic algae. Despite the limited research, EPA (2000) proposes boundaries of 20 mg/[m.sup.2] CHLA for oligotrophic to mesotrophic and 70 mg/[m.sup.2] CHLA for mesotrophic to eutrophic for mean periphytic algae. The mesotrophic to eutrophic boundary is relatively close to the 100 mg/[m.sup.2] CHLA proposed by Welch et al. (1988) for defining nuisance levels of periphytic algae.

An overabundance of macrophytes is also recognized as a potential water quality problem that is often overlooked in routine water quality assessments (Newton & Jarrell 1999). Macrophytes have clogged intakes for irrigation systems and can interfere with boating and fishing activities (Sosiak 2002). No current trophic guidelines exist for macrophytes, although management of marcrophytes is also important to maintaining healthy aquatic ecosystems (Chambers et al. 1999).

While instream planktonic CHLA concentrations are often monitored with regard to eutrophication, the excessive growth of periphyton and macrophytes are rarely considered with routine water quality monitoring, but may be just as important in defining impairment. The purpose of this study was to estimate the diversity and abundance (as biomass) of macrophytes and periphyton along the North Bosque River and its major tributaries and associate this information with measures of trophic status as indicators of stream eutrophication.

STUDY AREA

The North Bosque River is a fifth-order stream located in north central Texas that begins in Erath County and flows southeast for nearly 179 kilometers through Hamilton, Bosque, and McLennan counties until it enters Lake Waco near Waco, Texas(Figure 1). The mainstem of the North Bosque River for the most part is cut down to bedrock with permanent flow occurring primarily in the lower reaches and intermittent flow in the upper and mid-reaches. A small portion of the upper reach maintains a permanent flow due to discharge from the Stephenville wastewater treatment plant (WWTP). The flow associated with contributions from the Stephenville WWTP can dissipate within a few kilometers, particularly in the summer when rainfall-runoff events are infrequent and...

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