Do Nutrients Limit Algal Periphyton in Small Blackwater Coastal Plain Streams?

In this study, we examined the potential for nutrient limitation of algal periphyton biomass in blackwater streams draining the Georgia coastal plain.  Previous studies investigated nutrient limitation of planktonic algae in large blackwater rivers but virtually no scientific information existed regarding how algal periphyton respond to nutrients under different light conditions in smaller, low-flow streams.  Our objective was to determine if nutrients and/or light are factors limiting algal standing crop in streams draining a range of different land use types, from relatively undisturbed forest/wetland basins to highly disturbed agricultural sites.  We hypothesized that nutrient concentrations and light levels in sometimes heavily shaded coastal plain streams would not support nuisance algal biomass (chlorophyll a values between 100 and 200 mg m-2; Horner et al., 1983; Welch et al., 1988) even across different land uses.

Methods
Map of the southern coastal plain of Georgia showing study watersheds and nine blackwater stream sites.

Figure 1. Map of the southern coastal plain of Georgia showing study watersheds and nine blackwater stream sites.

We selected nine blackwater stream sites located within the Ocmulgee, Suwannee and Satilla river basins (Figure 1).

Human impacts within study stream basins ranged from relatively undisturbed (73-95% forest/wetland and 0.24-5% agriculture; n=4), to moderately disturbed (51–67% forest/wetland; 17-29% agriculture; n=3) to highly disturbed (34% forest/wetland; 41-54% agriculture; n=2).

We used a modification of the Matlock periphytometer (nutrient-diffusing substrata) (Figure 2) to determine if algal growth was nutrient- and/or light-limited at nine sites spanning a range of human impacts from relatively undisturbed forested basins to highly disturbed agricultural sites.  We employed four treatments in both shaded and sunny conditions (Figure 3 below) at each site:  (1) Control, (2) N (NO3-N), (3) P (PO4-P), and (4) N+P (NO3-N + PO4-P). Chlorophyll a response was measured on ten replicate substrates per treatment, after 15 days of in situ exposure.

Photographs of periphytometer modified from Matlock et al. (1998) showing primary frame with forty 20 ml scintillation vials and inset showing an inverted picture of primary and secondary frames together; when deployed, the secondary frame lies beneath the primary frame.

Figure 2. Photographs of periphytometer modified from Matlock et al. (1998) showing primary frame with forty 20 ml scintillation vials and inset showing an inverted picture of primary and secondary frames together; when deployed, the secondary frame lies beneath the primary frame.

Results

Our results strongly suggest that periphyton standing crop is primarily light-limited in heavily shaded coastal plain streams subjected to different land uses. Light conditions clearly affected chlorophyll a production: chlorophyll a levels for both controls and nutrient treatments were significantly lower in canopy-shaded conditions (p < 0.05; (Figures 4 and 5).

Modified periphytometers deployed in shaded and sunny conditions at Little Satilla Creek - Odum in the southern coastal plain of Georgia.

Figure 3. Modified periphytometers deployed in shaded and sunny conditions at Little Satilla Creek – Odum in the southern coastal plain of Georgia.

Nutrient concentrations can affect primary productivity but nutrient limitation of algal biomass generally occurs only if light conditions are favorable.  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.  Light-limiting effects of the blackwater itself may also inhibit periphyton growth in coastal plain streams . As light penetrates the water, high DOC concentrations and suspended solids can scatter and absorb light.

Chlorophyll a results from sun and shade at Station N - an intensively agricultural watershed and a tributary of the Little River. The results from the Sun treatment show a strong P response indicating that the stream was P limited. In contrast, the shade treatment shows no response indicating that shading is a controlling variable. Note the small standard deviation of the replicates indicating that this method is very robust.

Figures 4 and 5. Chlorophyll a results from sun and shade at Station N – an intensively agricultural watershed and a tributary of the Little River. The results from the Sun treatment show a strong P response indicating that the stream was P limited. In contrast, the shade treatment shows no response indicating that shading is a controlling variable. Note the small standard deviation of the replicates indicating that this method is very robust.

In summary, algal periphyton productivity in streams draining Georgia’s coastal plain appears to be primarily light-limited as a result of forested riparian zones and elevated DOC concentrations. Additionally, in light gaps and/or open areas receiving sunlight within these streams, algal periphyton growth can be secondarily nutrient-limited (most commonly by P and/or N+P). Chlorophyll a values from our study streams were well below nuisancelevels (100 – 200 mg m-2), even for nutrient-enriched treatments in sunny, nutrient-poor conditions and this may be attributed to natural blackwater characteristics. Finally, the statistical discriminating power of our periphytometer technique modified from Matlock et al. (1998) allows significant differences in algal standing crop to be accurately detected by decreasing the variability within treatment replicates, which could potentially increase the utility of nutrient enrichment studies in small streams by providing a clearer indication of treatment effects.


Carey, Richard O., George Vellidis, Richard Lowrance, and Catherine M. Pringle, 2007. Do Nutrients Limit Algal Periphyton in Small Blackwater Coastal Plain Streams? Journal of the American Water Resources Association (JAWRA) 43(5):1183-1193. Download .pdf

Download Richard Carey’s M.S. Thesis

Richard Carey, Ph.D. student, Agricultural & Biological Engineering Dept, University of Florida
George Vellidis, Professor; Biological and Agricultural Engineering, University of Georgia
Richard Lowrance, Ecologist, Southeast Watershed Research Laboratory, USDA-ARS, Tifton, Georgia
Catherine Pringle, Professor, Odum School of Ecology, University of Georgia