CADDIS Volume 2: Sources, Stressors & Responses
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Discharges of nutrients from point sources (pipes or canals) enter water bodies from discrete locations and may be continuous, making them easier to identify and monitor. They may be municipal or industrial effluents, confined animal feeding operation discharges, collected runoff from construction and development sites, or collected leachates from municipal landfills or waste disposal sites. In general, point sources which are not stormwater-related are relatively constant with respect to loadings (National Research Council 2000).
Nutrients in runoff and ground water enter waterbodies from their terrestrial watersheds. They may enter waterbodies diffusely from overland flow or groundwater discharge or at discrete locations, such as agricultural drainage tiles or stormwater outfalls. Moreover, accounting for and controlling these inputs is more difficult because they typically result from diffuse human activities across larger land areas than point sources. In addition, groundwater flow is typically unobserved and contributing source areas may be far from the actual receiving waters. Agriculture and urbanization in the upstream watershed are common alterations that are sources of nutrients. Silviculture, grazing, lawns and golf courses, may also produce runoff containing elevated nutrients. Both climate and physical characteristics of a watershed can increase the potential for nutrients to enter waterbodies via nonpoint source pathways. Land alterations generally increase nutrient delivery to streams, because they often enhance water flow across the landscape and mobilize nutrients that would otherwise be sequestered, thus increasing inputs to receiving waters. In addition, activities that disturb soils and thereby increase erosion also increase nutrient input to surface waters. The effect of such land alterations is further enhanced if soil concentrations of nutrients are increased by use of fertilizers, land application of manure or sludge, and leaking septic systems. High rainfall, steep slopes, and clay soils promote the transport of nitrogen and phosphorus via soil erosion and runoff, while low soil organic matter content and underlying sand, gravel, karst, or bedrock promote the transport of nitrogen to streams and rivers via groundwater (USGS 1999). More direct inputs of nutrients may be evidenced by steeply sloping banks or unfenced pastures.
Atmospheric deposition, either directly on the water body or on the watershed, can be an important source of nitrogen. The primary source of this deposited gaseous nitrogen is combustion of fossil fuels in power plants and motor vehicles. Deposition of nutrients in dust can be an important source in some regions and ecosystem types (e.g., Caribbean coral reefs).
Observations of aquatic plants may suggest that the excess nutrients are present. These observations could include prolific rooted emergent or floating macrophytes, or algae in the water column (i.e., phytoplankton) or attached to submerged surfaces (i.e., periphyton). The algae may be dominated by filamentous forms or may form algal mats (Figure 7) or blooms.
If the biological impairment is defined by changes in the aquatic plant community, nutrients should be considered as a candidate cause. Key biological parameters indicative of elevated nutrient concentrations include increased concentrations of chlorophyll a, changes to the structure of periphyton or algal assemblages, and a decrease in the abundance of submerged rooted macrophytes such as eel grass.
For animal assemblages, nutrients are generally not the proximate stressor, but often these assemblages change as a result of the effects on food or habitat. Increased phytoplankton may increase the abundance of filter feeders while increased periphyton may increase the abundance of scrapers. Increased macrophytes may favor fish that are ambush predators over those that are pursuit predators. Heavy growth of periphyton may decrease the habitat quality of gravel substrates for many species.
Algal blooms may include species that produce algal toxins. Fish kills or kills of macroinvertebrates suggest that algal toxins may be the cause of impairment.
Because nutrient enrichment often leads to decreases in dissolved oxygen concentrations, fish kills and other biological effects suggesting decreases in dissolved oxygen also may suggest high nutrient concentrations (see dissolved oxygen).
Specific biological measures that may be related to nutrients are discussed in the following subsections.
Chlorophyll a: Chlorophyll a (Chl a), a plant pigment produced by algae, is a common measure of algal biomass, and increases often indicate excess nutrients. There are several recognized methods for measuring Chl a, including spectrophotometric or fluorometric techniques. Sampling, filtration, and preservation techniques are important in the accuracy of these analyses (Carlson and Simpson 1996).
Algal and plant community structure: Periphyton includes all organisms (algae, bacteria, protozoans, and fungi) attached to submerged substrates. Periphytic algal communities have been used as indicators of water quality in freshwater ecosystems and indices are being developed for estuarine habitats. Periphyton can be collected by colonization of artificial substrates, such as periphytometers, or by direct sampling of natural substrates. Methods for characterizing periphytic assemblages employ characterizations of taxonomic structure or standing crop, including species identification and counts; relative abundance and dominance; community diversity, evenness, and similarity; biomass (i.e., ash-free dry mass, adenosine triphosphate content); pigment content (usually chlorophyll a); and biovolume (ASTM 1999, APHA et al. 1998; Table 1). Functional characteristics include measurements of photosynthesis (i.e., oxygen production, carbon assimilation), respiration (i.e., oxygen consumption), or enzymatic activity (e.g., alkaline phosphatase activity). The calculation and application of diversity and similarity indices to biological data, as well as other statistical techniques (e.g., principal components analysis or multi-dimensional scaling) have been summarized by Clarke and Warwick (1994), Legendre and Legendre (1998), and McCune et al. (2002). A review by Wetzel (1979) summarized problems associated with the collection and identification of periphyton that need to be considered during data interpretation, including substrate selectivity; poor differentiation between alive and dead or moribund individuals, particularly of diatoms; and sample replication and reproducibility.
|Periphyton metric||Expected response to increased nutrients||Description|
|Ash-free dry mass (mg/m²)||Increase||Ash-free dry mass of periphyton per unit area is a measure of the organic component of the periphyton.|
|Achnanthes minutissima (RA*)||Decrease||This cosmopolitan diatom species is relatively intolerant of high nutrients.|
|Eutrophic diatoms (RA* or richness)||Increase||These diatom taxa are identified as tolerant of eutrophic (i.e., high nutrient) conditions [see van Dam et al. (1994) for classifications].|
|Ratio, ACC to CMN diatoms||Decrease||The diatom genera Achnanthes, Cocconeis, and Cymbella are relatively intolerant of eutrophic conditions, while the genera Cyclotella, Melosira, and Nitzschia are relatively tolerant of eutrophic conditions.|
|Nitrogen-heterotrophic diatoms (RA*)||Increase||These diatom taxa can use or require organic nitrogen for their metabolism [see van Dam et al. (1994) for classifications].|
|Diatom tolerance value||Increase||If diatoms are scored using a descending scale, with hypereutrophic species (van Dam et al. 1994) scoring high (i.e., 3) and oligotrophic species scoring low (i.e., 1), the tolerance value is the average score for all diatoms identified in a sample.|
|Dominant diatom species (RA)||Increase||Under high nutrient conditions, one or a few diatom species often dominate the assemblage.|
*RA = relative abundance
In streams and rivers, periphytic algae live attached to submerged and benthic substrates. In extreme cases, a mat of periphyton may cover all benthic surfaces of a stream. In lakes, deleterious “algal blooms” may form at different times of the year. These blooms represent a significant alteration in the structure of the phytoplankton community, where one or a few species reproduce rapidly becoming both numerous and dominant (millions of cells per ml). In some cases, the dominating alga can produce toxins capable of killing fish or animals such as cattle or hogs that happen to drink the water. Examples of freshwater algae known to produce toxins include species of Anabaena and Cylindrospermopsis; some freshwater dinoflagellates also produce toxins. Blooms of toxin-producing algae are referred to as hazardous algal blooms.
In addition to increased algal or macrophyte standing crops, nutrient enrichment often changes the taxonomic structure of plant assemblages. If only phosphorus increases, nitrogen-fixing cyanobacteria (blue-green algae) may dominate phytoplankton assemblages in lakes, and in streams, chlorophytes (Chlorophyta or green algae) and cyanobacteria (Cyanophyta or blue-green algae) may dominate over diatoms. These changes in taxonomic structure also may result in changes in cell size or palatability, which can affect planktonic-feeding zooplankton in lakes or algal-feeding grazers in streams. In addition, increases in phytoplankton can raise turbidity and reduce penetration of light, potentially leading to decreases in benthic flora, such as submerged aquatic vegetation.
Macroinvertebrates and Fish: Benthic macroinvertebrate communities consist of bottom-dwelling aquatic insects and other invertebrates that occur in streams, rivers or lakes. These communities can be useful indicators of anthropogenic stress, as the composition and density of the macroinvertebrate taxa are a function of water quality during the recent past, including random discharges of pollutants that may not be detected by water quality monitoring. In addition, these organisms survive and thrive under a wide range of environmental conditions, are easily collected, and are associated with well-documented and readily availably assessment methodologies. Some community metrics may suggest the type of stressor(s) impacting an aquatic system, or serve as more general indicators of stress (Table 2).
An increase in algal growth resulting from increased nutrient loadings can affect the food webs of aquatic ecosystems, including the macrobenthic component. For example, increased productivity of periphyton in small streams will shift the macrobenthic community to one that is dominated by species, such as gastropod snails and other algal grazers and scrapers, that use algal mats for food and habitat. Increased algal growth also means increased algal decomposition and increased production of fine organic particles, which, in turn, may greatly increase the densities of filter-feeding macrobenthos, such as the larvae of black flies.
|Macroinvertebrate metric||Expected response to increased nutrients||Description|
|EPT taxa (RA or richness)||Decrease||Ephemeroptera (mayfly), Plecoptera (stonefly), and Trichoptera (caddisfly) larvae represent taxa that often decrease with increases in many types of stress, including nutrient enrichment (Klemm et al. 2003).|
|Mollusca and Crustacea (RA or richness)||Increase||Mollusks, such as gastropod snails and fingernail clams, and crustaceans, such as amphipods or isopods are tolerant of nutrient enrichment (Griffith et al. 2005).|
|Most dominant taxa (RA)||Increase||Reduced diversity and uneven distribution of individuals among taxa is characteristic of increased stress, including nutrient enrichment (Barbour et al. 1996).|
|Tanytarsini (RA or richness)||Decrease||This tribe of Chironomidae is generally considered intermediate in tolerance to stressors, such as nutrients (Deshon 1995).|
|Grazers and Scrapers (RA or richness)||Increase||Grazers and Scrapers feed on the increased algal production associated with nutrient enrichment (Barbour et al. 1999).|
|Sensitive species (RA or richness)||Decrease||Sensitive species decrease in response to increased stress, including nutrient enrichment (Miltner and Rankin 1998).|
|Tolerant species (RA)||Increase||More tolerant species may increase in abundance in response to decreases in sensitive species as result of nutrient enrichment (Miltner and Rankin 1998).|
Excess macrophytes or algae can alter habitats for both fish and macroinvertebrates by taking up space and altering substrate surfaces. Excess algae can contribute to turbidity, which interferes with sight-feeding fish (see Sediments module).
There are no site observations that specifically provide evidence of the absence of nutrients. General reasons for excluding a candidate from the list are described in Step 2 of the Step-by-Step guide and in Tips for Listing Candidate Causes.
We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding nutrients from your initial list of candidate causes, because different species have different nutrient requirements and different sites have different naturally occurring levels of nutrients.