Productivity In Aquatic Systems Research Articles

A decision support system for phosphorus management at a watershed scale.

Phosphorus (P) is one of the main nutrients controlling algal production in aquatic systems. Proper management of P in agricultural production systems can greatly enhance our ability to combat pollution of aquatic environments. To address this issue, a decision support system (DSS) consisting of the Maryland Phosphorus Index (PI), diagnosis expert system (ES), prescription ES, and a nonpoint-source pollution model, Ground Water Loading Effects of Agricultural Management Systems (GLEAMS), was developed and applied to an agricultural watershed in southern Sweden. This system can identify critical source areas (CSAs) regarding phosphorus losses within the watershed, make a diagnosis of probable causes, prescribe the most appropriate best management practices (BMPs), and test the environmental effects of the applied BMPs. The PI calculations identified small parts of the watershed as CSAs. Only 10.4% of the total watershed area in 1995 and 5.2% of the total watershed area in 1996 were classed as "high potential P movement." Four probable causes (high P level in soil, excessive P fertilization, stream proximity, and subsurface drainage) and three BMPs (riparian buffer strips, reduced P fertilizer application, and P fertilizer incorporation) were identified by a diagnosis and prescription expert system. The GLEAMS simulations conducted for one selected CSA field for a 24-yr period showed that the recommended BMP reduced runoff P losses by 55% and sediment P losses by 71%, if applied from the first year. Results showed that using DSS may enable us to select a proper BMP implementation strategy and to realize the beneficial effect of BMPs on a long-term basis.

Using stable isotopes to monitor watershed influences on aquatic trophodynamics

Stable C and N isotope measurements of the clam Potamocorbula amurensis were used to help identify watershed-level differences in food webs of San Francisco Bay. Potamocorbula amurensis has become widely distributed in San Francisco Bay since introduction from Asia in1986. Clam samples were collected from both the river-influenced northern arm of San Francisco Bay and the lagoonal southern arm of the Bay during 1990-1991. Carbon isotopic compositions of clams responded primarily to riverine inputs and provided an index of hydrologic mixing across the estuarine system. Nitrogen isotopic compositions of clams were more responsive to watershed nutrient loading, with higher δ15N values found in South Bay, which receives stronger inputs of anthropogenic N. Routine monitoring of animal consumer isotopic compositions could be an effective way to detect long-term watershed-level changes in C and N dynamics important for secondary production in aquatic systems.

Large-Scale Trends for Stream Benthic Respiration

Large-Scale Trends for Stream Benthic Respiration

Stimulation of aquatic bacterial activity by cyanobacteria

The time-course response of natural bacterial populations and isolates from lake water to various densities of the filamentous cyanobacteriaAphanizomenon flos-aquae andLyngbya birgei collected from the same lake is reported. The cyanobacteria were separated from the bacteria by dialysis membranes that allowed only dissolved cyanobacterial products to pass. Bacterial3H-thymidine incorporation and cell number were significantly (p<0.05) correlated with cyanobacterial density for both species. Estimated dissolved organic carbon (DOC) utilization, based on bacterial biomass changes over time, were usually significantly (p<0.01) correlated with cyanobacterial density and the decrease in bulk pool DOC for both species. Bacterial volume per cell increased significantly (p<0.05) in response to cyanobacterial density on day 5 of the experiments; cell volume remained unchanged on day 1. Bacterial cell numbers on outer surfaces of the tubular membrane containing the cyanobacteria (on the side exposed to the test bacteria) were significantly (p<0.01) correlated with cyanobacterial density. Statistical analysis inferred that bacteria closely associated with cyanobacteria (i.e. attached) responded more strongly to cyanobacterial products than free-living bacteria. Overall, our results indicate that cyanobacterial products have a potentially important role in regulating bacterioplankton productivity in aquatic systems.

Estimates of bacterial growth rate constants from thymidine incorporation and variable conversion factors

Thymidine incorporation into DNA is widely used to estimate rates of bacterial growth and secondary production in aquatic systems. The procedure requires the use of several conversion factors and assumptions to convert rates of thymidine uptake to rates of carbon production. Perhaps the most controversial of the conversion factors is that which converts rates of incorporation to the rate of cell production. During a year-long study in Lake Arlington, Texas, we empirically determined conversion factors from bacterial growth in filtered (1.0 μm porosity) and diluted (1:9) lake water. Bacterial growth rate constants determined from changes in cell abundance were compared to growth rate constants estimated from empirically derived (both instantaneous and annually averaged), theoretical, and modeled conversion factors. Single value conversion factors (i.e., theoretical, or average of 19 empirically determined conversion factors) did not yield estimates of bacterial growth rate constants that compared favorably to growth estimates from changes in bacterial abundance. Conversion factors, determined from a regression model based on empirically determined conversion factors, gave annual growth estimates that were similar to those obtained from changes in cell abundance.

Evidence for sulphate-controlled phosphorus release from sediments of aquatic systems

SEDIMENTS of aquatic systems play a critical part in controlling phosphorus loading to the water column1. Because P loading is an important determinant of productivity in aquatic systems, there has been keen interest in variables that influence P release from sediments. In disagreement with present theories1,2 our data from 23 different aquatic systems indicate that sulphate concentration of waters is an extremely important variable controlling P release from sediments. The increased P release from sediments at higher sulphate concentrations may help to explain why primary production in freshwater systems (with relatively low sulphate concentrations) tends to be P limited3, whereas in many saline systems (with high sulphate concentrations) production is often P sufficient4. Further, our results indicate that anthropogenically induced changes from atmospheric S inputs could, over time, alter the P cycle of aquatic systems.

Photosynthetic strategies and productivity in aquatic systems

Aquatic plants have physiological, morphological and growth-form adaptations that collectively determine a species' success in a given habitat. The primary productivity in various aquatic habitats is largely dependent on the constraints on both species composition and photosynthetic rates, thus, the characteristics of the assembled species should reflect such constraints. For the macrophytes, major adaptations for life in water are associated with constraints on photosynthesis: adaptations to increase carbon fixation, for example, include; (1) mechanisms to concentrate inorganic carbon for photosynthesis, such as C 4-like and CAM carbon fixation, and the ability to remove HCO 3 − from the water; (2) the use of alternate sources of carbon, such as sediment CO 2; (3) adaptation for net photosynthesis at low-light intensities; (4) morphological adaptation to increase inorganic carbon or mineral acquisition. We evaluate such characteristics at the species or community level, to provide insight into the ways in which adaptations for photosynthesis are integrated into a plant's strategy or suite of adaptations to constraints on growth or production. In nutrient-poor, softwater habitats, for example, the common rooted plants have adaptations that allow tolerance of low rates of nutrient delivery to the plant and the conservation of captured resources (e.g. small evergreen plants with inherently slow growth that use sediment CO 2 and CAM). These adaptations are compatible with the low rates of nutrient availability, although the slow growth results in low annual primary production. In habitats with generally higher levels of inorganic carbon and other nutrients, where competition among plants limits the availability of these resources, species have adaptations that allow rapid growth and biomass accumulation. Here, species are more likely to be able to remove HCO 3 − from the water for photosynthesis efficiently, and may have other carbon-concentrating mechanisms that allow rapid photosynthesis. These considerations serve as one possible framework for relating adaptations for photosynthesis to plant growth characteristics and the primary productivity of aquatic systems.

FLORIDA LAKES ASSESSMENT: COMBINING MACROPHYTE, CHLOROPHYLL, NUTRIENT, AND PUBLIC BENEFIT PARAMETERS INTO A MEANINGFUL LAKE MANAGEMENT SCHEME

ABSTRACT Numerous indices developed during the last 10 years to quantify the concept of trophic state turned out to be too ambiguous to be useful to lake managers. Based on empirical relationships between various water quality indicators, the most widely used indices use Secchi disk transparency, chlorophyll a concentration, and total phosphorus as a measure of lake trophic state. Indices based on data from north temperate lakes, however, are not directly applicable to Florida lakes. Many Florida lakes are known to be nitrogen rather than phosphorus limited. Florida lakes also commonly have macrophyte problems which are not accounted for by such indices. A series of indices based on Secchi disk transparency, chlorophyll a concentration, macrophyte abundance, total phosphorus concentration, and total nitrogen concentration were developed for Florida lakes. The Secchi disk index serves as a physical measure of trophic state. The public perceives water clarity as an important attribute of lakes, and Secchi disk transparency is a good measure of water clarity. The biological measures of trophic state are based on chlorophyll a concentration and macrophyte coverage. Chlorophyll a concentration is a good indicator of algal populations and macrophyte coverage is related to potential aquatic weed problems. Since phytoplankton and macrophytes compete for similar habitats, Florida's shallow lakes usually do not experience nuisance conditions of both these autotrophs simultaneously. The chemical measures of the trophic state are total phosphorus and total nitrogen concentrations. These elements are the principal nutrients limiting primary productivity in aquatic systems and therefore provide useful information on the nutritional status of lakes. When the lake is primarily phosphorus limited, based on the ratio of total nitrogen to total phosphorus concentration, the phosphorus trophic index is used. If the lake is nitrogen limited, based on the concentration ratio, the nitrogen index is used. Many Florida lakes are not limited by a single nutrient and are relatively well balanced. In this case an average of the nitrogen and phosphorus indices is used to determine the trophic status. The overall trophic state index for a Florida lake is determined by combining the appropriate values obtained from the physical, chemical, and biological indices. Over 570 lakes in the State were ranked by this method. The relative simplicity of the trophic index, combined with its accuracy and reliability, emphasizes its utility in statewide lake management.

Microorganisms and Interfaces

Microorganisms are, by definition, the smallest of all living organisms, yet they play a dominant role in primary productivity in aquatic systems, in decomposition of waste organic materials, and in biogeochemical transformations of mineral elements in both terrestrial and aquatic systems (Alexander 1971). All microorganisms are dependent upon inorganic nutrients and, in the case of heterotrophs, organic nutrients for satisfactory growth and metabolism. Some natural microbial habitats are characterized by a relatively constant input of nutrients, which sustain a high level of microbial activity; but many habitats receive only spasmodic or chronically low nutrient inputs, resulting in relatively low microbial activity. Such habitats include many soils, the open oceans, and oligotrophic lakes and streams. The majority of microorganisms require an aqueous environment for growth. Where the aqueous phase is nutrient-deficient, substantial microbial activity usually is encountered only at interfaces in the system. Interfaces are boundaries between any two phases in a heterogeneous system and possessing physicochemical properties that differ from those of either phase. Natural microbial habitats are heterogeneous systems containing a wide range of solid-liquid, gas-liquid, liquid-liquid, and, occasionally, solid-gas interfaces. Such interfaces serve as sites of nutrient accumulation in the form of lipids, macromolecules, other organic molecules, and inorganic nutrients. Microorganisms tend to accumulate at solidliquid interfaces (Stark et al. 1938, ZoBell and Anderson 1936) and gas-liquid interfaces (Parker and Barsom 1970). They achieve substantial growth at these interfaces even in highly oligotrophic conditions. The interfaces can be regarded as sites of escape from the nutrient-deficient aqueous phase; thus, all interfaces exert a potential influence on microbial distribution, growth, metabolism, and succession in such habitats. The solid phases found in nature include large surfaces, such as rocks, ship hulls, plants, and animals, as well as small surfaces, such as clay particles, plankton, and biological debris suspended in the aqueous phase. The major gas phase is air at the surface of large water bodies, but this phase also may consist of H2, CO2, CH4, and H2S in anaerobic sediments and soils, as well as in gas bubbles originating from sediments. The most obvious liquid-liquid interface is that encountered following an oil spill in oceans and estuaries, where the oil phase overlies the aqueous phase. Although interfaces provide an advantageous situation with respect to the growth of microorganisms, there are instances where microbial activity at such sites is of nuisance value to man. Microbial growth on the inside of pipelines and on ship surfaces, for example, increases surface roughness and thus creates greater fluid frictional resistance; microbial growth on heat transfer surfaces has an insulating effect that alters the heat transfer characteristics of the system (Characklis 1980). A study of microorganisms and interfaces, then, must consider the modes of transport of the organisms to interfaces and the physicochemical factors involved in interactions between the microorganisms and the interfaces. Recent research has concentrated mainly on the behavior of bacteria at interfaces, because bacteria colonize interfaces very rapidly. The smallest of all living organisms (most range from 0.5 to 2.0 /tm in length), bacteria possess a net negative surface charge and tend to behave as colloidal particles in an aqueous phase. Critical studies of both the biological and colloidal properties of bacteria (Marshall 1976) have substantially expanded the information on bacteria at interfaces. The surface charge properties of bacteria may be altered by lowering the pH of the aqueous phase or by adding polyvalent cations (Santoro and Stotzky 1967), as well as by adsorption of organic materials to the bacterial surface (Neihof and Loeb 1974).