Submitted to: Journal of the American Water Resources Association
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/5/2007
Publication Date: 10/10/2007
Citation: Carey, R.O., Vellidis, G., Lowrance, R.R., Pringle, C.M. 2007. Do nutrients limit algal periphyton in blackwater coastal plain streams? Journal of the American Water Resources Association. 43:1183-1193. Interpretive Summary: Low dissolved oxygen (DO) levels in coastal plain streams are a major reason that these streams do not meet water quality standards set by state and federal regulatory agencies. The low dissolved oxygen may be due to excess nutrients which derived from agricultural non point source pollution. We tested whether excess nutrients would increase the growth of periphyton (algae that grows on permanent surfaces in the stream such as logs and rocks). The effects of added nutrients were tested in both full sun and in shaded conditions. In shaded conditions, there was no effect of adding nutrients in 6 of 9 streams, indicating that algae growth was limited by shade. In sunlight, increased nutrients caused increased algae growth at most sites. Of nine streams tested, six showed a response to either phosphorus addition or phosphorus plus nitrogen addition in full sunlight. For all streams, there was significantly more growth of algae in areas receiving more sunlight than in areas that were more shaded. The chlorophyll levels developed by the growing algae were low compared to nuisance levels. If coastal plain streams have intact riparian forest buffers that provide shade for the stream channel, it is unlikely that nutrient additions will increase periphyton growth. The nutrient responses in full sun indicate that the streams have levels of nutrients below that needed for maximum periphyton growth and indicate that periphyton growth is not currently affected by excess nonpoint source nutrients. The results show that periphyton growth is not currently stimulated by excess nutrients and it is unlikely that this process will contribute to low dissolved oxygen conditions. If more nutrients enter the streams, it is possible that increased periphyton growth would occur where streams are in full sunlight.
Technical Abstract: Many streams draining the U.S. Atlantic coastal plain are listed as having impaired water quality because of low levels of dissolved oxygen (DO). Nutrient enrichment from nonpoint source pollution, and associated algal and bacterial growth (which create a biological oxygen demand), may be the causes of low DO. We examined the potential for nutrient limitation of algal periphyton growth in blackwater streams in the Georgia coastal plain. We employed nutrient-diffusing substrata as a bioassay to determine if algal growth was nutrient- and/or light-limited at eight sites with < 5 mg DO L-1 . These eight sites spanned a range of human impacts from relatively undisturbed forested basins to highly disturbed agricultural sites. We also included one un-impaired reference site with atypically high DO levels (> 5 mg DO L-1). We employed four treatments in both shaded and sunny conditions at each site: (1) Control, (2) N (NO3-N), (3) P (PO4-P), and (4) N+P (NO3-N + PO4-P). Algal standing crop was light-limited in typical forested conditions at all nine sites: control treatments at canopy-shaded sites were significantly (P < 0.05) lower than those in the sun. Nutrient limitation (N, P, and/or N+P) of algal standing crop in the shade was only significant (P < 0.05) at two of the impaired sites and at our reference site. In contrast, in sunny conditions, algal standing crop was significantly (P < 0.05) nutrient-limited at five of the listed impaired sites and at the reference site. The lack of algal response to nutrients in sunny conditions at two Suwannoochee Creek sites may be due to high levels of dissolved organic carbon (41-52 mg DOC L-1) and consequent light-limiting effects of the blackwater itself. For Georgia coastal plain streams that are listed as being DO-impaired, algal periphyton growth appears to be primarily light-limited and can be secondarily nutrient-limited (most commonly by P or N+P combined) in light gaps and/or open areas receiving sunlight. Chlorophyll a values did not approach what have been defined as nuisance levels (i.e. 100 - 200 mg m-2), even in response to nutrient enrichment in sunny conditions. Highest chlorophyll a levels (70 mg m-2) were found in an agricultural stream in response to N+P enrichment but values were generally well below 50 mg chlorophyll a m-2.