Grantee Research Project Results

Final Report: Microbial indicators of biological integrity and nutrient stress for aquatic ecosystems

EPA Grant Number: R825868
Title: Microbial indicators of biological integrity and nutrient stress for aquatic ecosystems
Investigators: Grover, James P. , Chrzanowski, Thomas H.
Institution: The University of Texas at Arlington
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 1997 through August 31, 2000
Project Amount: $748,000
RFA: Ecosystem Indicators (1997) RFA Text |  Recipients Lists
Research Category: Aquatic Ecosystems , Ecological Indicators/Assessment/Restoration

Objective:

Our research explored connections between biological integrity and nutrient supply and limitation, focusing on the microbial component of lakes and reservoirs. Microbial indicators should respond rapidly to increased nutrient supply and alterations in supply ratios of different nutrients. We examined two reservoirs in north Texas year-round and two lakes in the Experimental Lakes Area (Ontario, Canada) during the ice-free growing season. One of the latter lakes is pristine, while the other has been experimentally eutrophied with phosphorus additions for about 30 years. We studied indicators emphasizing connections between processing of materials (e.g., nutrients) and the population and community dynamics of microorganisms. We focused on the following: particulate chemistry, particularly carbon (C), nitrogen (N), and phosphorus (P) composition; responses of algae and bacteria to dilution bioassays of nutrient limitation; community structure and diversity of algae in relation to shifting nutrient limitation; community-level physiology of bacteria in relation to shifting nutrient limitation; and relationships among nutrient limitation, intensity of grazing on bacteria, and relative productivity of algae and bacteria. We tested several working hypotheses concerning patterns and correlations among these indicators, and assessed their seasonal and between-lake variability.

Summary/Accomplishments (Outputs/Outcomes):

In general, the various indicators of algal and microbial function that we examined appear useful at different scales. Nearly all displayed strong seasonal or temporal variation, and thus potentially illuminate ecological dynamics within lakes. Many quantities also displayed consistent differences between lakes of different trophic status, making them potential indicators of algal and microbial functions for comparisons between lakes. Within-lake variations were generally more pronounced in Texas lakes due to the long growing season and our year-round sampling schedule, and we focused on these lakes for many of the quantities that were studied. Between-lake comparisons are based on a more eutrophic and a less eutrophic lake within each of two regions: Texas and Canada.

Hypothesis 1: Stoichiometric indicators of nutrient limitation based on the C:N:P ratio of seston, and bioassay indicators of nutrient limitation will agree.

At the within-lake temporal and seasonal scale, there is agreement among the particulate C:P ratio, particulate N:P ratio, and results of bioassay experiments as indicators of P-limitation. Results of bioassay experiments do not agree with particulate C:N and N:P ratios as indicators of algal N-limitation. When both N and P are limiting, the particulate N:P ratio seems more responsive to P-limitation. The bioassay method did not provide reliable data from the Canadian lakes. In applying the bioassay method, results should be scrutinized to eliminate experiments where the method seems to fail. At the scale of between-lake comparisons: particulate C:P and particulate N:P agree with other trophic status indicators and with the expectation that P-limitation is stronger in less eutrophic lakes, and N-limitation in more eutrophic lakes. The particulate C:N ratio does not agree with these expectations, probably due to greater activity of N-fixing cyanobacteria in eutrophic lakes. Results of bioassay experiments in Texas on average reveal differences in P- and N-limitation between a less eutrophic and a more eutrophic lake, but there also is wide overlap between lakes. Where the experimental technique succeeds, these bioassay indicators appear more useful for documenting seasonality and timing of nutrient limitation than for comparing nutrient limitation between lakes.

Hypotheses 2 and 3: If a single nutrient is limiting, the degree of algal nutrient limitation will be negatively associated with algal community diversity and equitability in situ. If two nutrients are limiting, the degree of algal nutrient limitation will be positively associated with algal community diversity and equitability in situ.

At the within-lake scale, algal diversity is associated with seasonal changes in the number of limiting resources. This pattern is consistent with the theory that inspired these hypotheses, which predicts that limitation by multiple, rather than single resources, promotes diversity. In comparing a more eutrophic to a less eutrophic lake, a unit increase in the number of limiting resources increased diversity about twice as much in the less eutrophic lake.

Hypothesis 4: Bacterial community structure will shift in response to nutrient limitation.

Our technique, based on substrate utilization in Biolog? assays, documented potential changes in bacterial community function, but indicates structure indirectly at best. Its interpretation is sufficiently controversial, and the results summarized should be verified with alternative techniques. To quantify metabolic shifts among bacteria, we developed an index of carbohydrate utilization relative to utilization of other substrates. Bacteria shift progressively to carbohydrate utilization and away from utilization of other substrates during the long growing season in Texas, with an opposite shift in winter. At the scale of between-lake comparisons, the growing season average of the index of carbohydrate utilization is positively correlated with the trophic status of different lakes. We propose that greater carbohydrate metabolism of bacteria during periods of high algal productivity is a general feature of eutrophic lakes.

Hypothesis 5: Bacterial nutrient limitation will be negatively associated with estimated loss to micrograzers.

This hypothesis was not supported. At the within-lake scale, the degree of bacterial nutrient-limitation in Texas lakes was uncorrelated with estimated mortality to micrograzers. At the scale of between-lake comparisons, relationships between bacterial mortality rate and temperature were statistically indistinguishable between a more eutrophic and a less eutrophic lake. A preliminary assessment of grazing mortality of phytoplankton in the Texas lakes suggests that micrograzers (rotifers and nauplii) are the only significant grazers in eutrophic Eagle Mountain Lake, while cladocera are significant grazers in mesotrophic Joe Pool Lake during cool seasons.

Hypothesis 6: Degrees of algal and bacterial nutrient limitation will be positively associated, implying competition for the same nutrient pool, especially when P is identified as a limiting nutrient.

At the within-lake scale, this hypothesis was confirmed. Seasonal variations of algal and bacterial nutrient limitation were documented in the Texas lakes. Resource-saturated growth rates of both groups increase strongly with temperature; resource-limited growth rates either increase weakly with temperature (bacteria) or decrease (algae). These relationships with temperature imply that both groups are nutrient-limited during warm weather (water temperature >20?C), but generally not during winter (water temperature <12?C). N- and P-limitation frequently co-occurred in bacteria and algae, and often depressed the growth rate of algae more severely than that of bacteria, suggesting that bacteria are usually better competitors for nutrients. Relationships between nutrient-saturated and nutrient-limited growth rates of algae and bacteria with temperature were statistically indistinguishable between a more eutrophic and a less eutrophic lake. Responses of bacteria to N and P additions in bioassay experiments were also similar between these lakes. Thus, the fine-scale variations in nutrient-limitation of bacteria do not seem to indicate large differences between more and less eutrophic lakes.

Hypothesis 7: The degree of algal nutrient limitation will be negatively associated with the estimated P/R ratio, and this association will be strongest when P limits algal growth, due to superior bacterial competition. We developed estimates of algal and bacterial productivity from bioassay experiments and abundance data.

We then used the ratio of algal:bacterial productivity as a surrogate for the ecosystem-level production:respiration ratio (P/R), on the premise that algal and bacterial activities strongly influence this latter ratio in pelagic systems. At the within-lake scale, seasonal variations in algal:bacterial productivity were not related to indicators of P-limitation (SRP or particulate C:P ratio) as hypothesized. At the scale of between-lake comparisons, the ratio of algal:bacterial productivity was higher on average in a more eutrophic lake than in a less eutrophic lake. This is consistent with the hypothesis, given that P-limitation of algae was stronger and more frequent in the less eutrophic lake.

Table 1. Potential indicators for comparing algal and microbial resource limitation within and between lakes

Quantity	Diagnostic criterion for between-lake comparisons	Diagnostic criterion for within-lake studies	Comments
Particulate C:P	Lower in eutrophic lakes	Higher during periods of P-limitation	Easily determined with appropriate equipment (CHN analyzer and bench chemistry)
Particulate C:N	Lower in eutrophic lakes	Use for this purpose not indicated	Easily determined with appropriate equipment (CHN analyzer)
Particulate N:P	Lower in eutrophic lakes	Higher during periods of P-limitation	Easily determined with appropriate equipment (CHN analyzer and bench chemistry)
Total and simple effects of N or P additions in algal bioassay experiments	Lower extremes in eutrophic lakes (P); higher in eutrophic lakes (N)	Higher during periods of N- or P-limitation	Technique fails in some lakes, is moderately labor-intensive, confirmation of utility requires comparative study of more lakes
Nutrient-saturated and nutrient-limited algal and bacterial growth rates from bioassay experiments	Use for this purpose not indicated	Ratio of nutrient-limited to nutrient-saturated growth rate lower during periods of nutrient limitation	Technique fails in some lakes, is moderately labor-intensive, confirmation of utility requires comparative study of more lakes
Algal diversity	Weaker relationship to number of limiting resources in eutrophic lakes	Higher during periods of multiple resource limitation	Very labor-intensive, technique requires statistical development and evaluation, confirmation of utility requires comparative study of more lakes
Bacterial carbohydrate utilization in Biolog? assays	Higher during growing season in eutrophic lakes	Higher during periods of algal nutrient limitation	Easily determined with appropriate equipment (Biolog plates and reader), controversial technique, requires confirmation by other techniques
Ratio of algal:bacterial productivity determined from bioassay experiments	Higher in eutrophic lakes	Use for this purpose not indicated	Bioassay technique fails in some lakes, is moderately labor-intensive, should be compared to alternative tracer methods

By addressing the working hypotheses considered above, we developed some judgments about several of the quantities that we measured as potential indicators of algal and microbial function in lakes of different trophic status (table above). When making comparisons of trophic status between lakes, there are already widely used indicators with proven track records such as chlorophyll a, Secchi depth, and TP. The quantities highlighted could potentially provide corroborative evidence of trophic status, and offer additional insights beyond those of conventional indicators. These include information on relative degrees of N and P limitation, and on algal and bacterial functions that affect the production and decomposition of organic matter. This latter issue could be especially relevant to the growing concern with total and dissolved organic carbon in drinking water supplies, due to the tendency to form halogenated compounds during treatment. All of the quantities we evaluate as potential indicators for comparisons between lakes vary seasonally, and thus growing-season or annual averages must be used to standardize any comparisons. The seasonal variation of many quantities implies that they also can be used within lakes to identify periods of nutrient limitation and associated changes in algal and bacterial function. As a general rule, nutrient limitation is most likely during the growing season, but timing and strength may vary within and between lakes. This issue may be especially important in lakes in warmer climates with long growing seasons.

Journal Articles on this Report : 3 Displayed | Download in RIS Format

Publications Views

Other project views:	All 25 publications	3 publications in selected types	All 3 journal articles

Publications

Type	Citation	Project	Document Sources
Journal Article	Chrzanowski TH, Grover JP. Effects of mineral nutrients on the growth of bacterio- and phytoplankton in two southern reservoirs. Limnology and Oceanography 2001;46(6):1319-1330.	R825868 (1999) R825868 (2000) R825868 (Final)	Abstract: ASLO Abstract Exit Other: ASLO PDF Exit
Journal Article	Chrzanowski TH, Grover JP. The light:nutrient ratio in lakes: a test of hypothesized trends in bacterial nutrient limitation. Ecology Letters 2001;4(5):453-457.	R825868 (Final)	Abstract: Blackwell-Synergy Abstract Exit
Journal Article	Grover JP, Chrzanowski TH. Seasonal patterns of substrate utilization by bacterioplankton: case studies in four temperate lakes of different latitudes. Aquatic Microbial Ecology 2000;23(1):41-54.	R825868 (2000) R825868 (Final)	Abstract: IR Abstract Exit Other: IR PDF Exit

Supplemental Keywords:

water, ecological effects, metabolism, ecosystem, indicators, ecology, limnology, analytical, Central, Texas, TX, Canada., RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Water, Aquatic Ecosystem, Aquatic Ecosystems & Estuarine Research, Ecological Indicators, Ecological Risk Assessment, Ecosystem Protection, Aquatic Ecosystem Restoration, Ecosystem/Assessment/Indicators, exploratory research environmental biology, Microbiology, Ecological Effects - Environmental Exposure & Risk, Limnology, Nutrients, environmental indicators, bioavailability, demographic factors, ecological condition, ecological effects, ecosystem health, lakes, nutrient stress, water quality, aquatic biota , ecosystem assessment, ecosystem condition, Seston C:N:P ratio, nutrient loading, algae, ecosystem, interactive aquatic ecosystem indicator, interactive stressors, microbial, wetland eutrophication, biological integrity, aquatic, biotic integrity, ecosystem indicators, wetland habitat, eutrophication, adverse impacts, biological activity, environmental stress, hydrological stability, ecological exposure, ecological stoichiometry, algal growth, nitrogen, aquatic ecosystems, molecular detection, biological indicators, nutrient management

Relevant Websites:

http://www.uta.edu/biology/chrzanowski/index.html
http://www.uta.edu/biology/grover/index.htm
http://www.aslo.org/lo/toc/vol_46/issue_6/1319a1.pdf

Progress and Final Reports:

Original Abstract

1998

1999 Progress Report