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CADDIS Volume 2: Sources, Stressors & Responses


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Other sources/stressors/responses



Authors: J.C. Kurtz, M.B. Griffith, M.A. Morrison, C.E. Hornig

A cyanobacterium (Nodularia spumigens) bloom.  Photo by J.D. Kinnon, Wood's Hole Oceanographic Institute, http://www.whoi.edu/redtide/rtphotos/rtphotos.html
Figure 1. A cyanobacterium (Nodularia spumigens) bloom in the Gippsland Lakes, Victoria, Australia in January, 2002.
Courtesy of J.D. Kinnon, Wood's Hole Oceanographic Institute.

Nutrients are elements that are essential for plant growth, including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), and silicon (Si). N, P, and K are considered primary nutrients, and N and P are the major limiting nutrients in most aquatic environments.

When considering candidate causes, N and P are evaluated to determine the trophic status (or relative nutrient condition) of freshwater systems. In lakes, streams, large rivers, and estuaries, trophic status may be expressed in terms of oligotrophy (low nutrients, minimally productive), mesotrophy (moderate nutrients, moderately productive), or eutrophy (high nutrients, highly productive).

In most cases, nutrients are not proximate stressors for aquatic communities. Although certain forms of N [i.e., unionized ammonia (NH3), nitrite (NO2-) and, in some cases, nitrate (NO3-)] may be toxic, these effects are considered in the ammonia and unspecified toxic chemicals modules. Nutrients have indirect adverse effects on aquatic communities through their effects on primary production, the growth and accumulation of plant and algal biomass, and the species composition of algae (i.e., phytoplankton in lakes or periphyton in streams) and other plant assemblages (Figures 1 and 2); Dodds and Welch 2000).

Large mats of submerged aquatic vegetation. Photo by Christy A. Crandall, USGS, http://pubs.usgs.gov/circ/circ1225/images/p19photo11.html
Figure 2. Large mats of submerged aquatic vegetation.
Courtesy of C.A. Crandall, USGS.

Increasing primary production and changes in plant species composition can be proximate causes of effects on consumers (i.e., macroinvertebrates and fish) by:

  • Altering food resources: the amount of food resources, their type (e.g., living plant and algal biomass versus detritus),or their palatability (e.g., changes in cell size in algae for filter-feeding animals);
  • Altering habitat structure: changes in benthic interstitial space, ease of movement across benthic surfaces or through the water column and availability of macrophytes as habitat for some species and life stages; and
  • Algal toxins: some algae characteristic of eutrophic conditions can be toxic to fish, invertebrates, and even humans.

These increases in primary production and plant and algal biomass can affect other physical and chemical characteristics of the water body, such as pH and dissolved oxygen, which in turn may be proximate stressors for the aquatic community. Algal growth and biomass accumulation associated with excess nutrients also hinder recreation, fishing, hunting, and aesthetic enjoyment of waterbodies and may interfere with drinking water treatment and use of water by industrial facilities.

Simplified conceptual model for nutrients
Figure 3. A simple conceptual diagram illustrating causal pathways, from sources to impairments, related to nutrients. Click on the diagram to go to the Conceptual Diagrams tab and view a larger version.

Checklist of sources, site evidence and biological effects

This module addresses excess nutrients as a stressor leading to plant assemblage changes and other proximate stressors (e.g., low dissolved oxygen); toxic effects of nutrients are considered in the ammonia and unspecified toxic chemicals modules. Nutrients should be listed as a candidate cause when potential human sources and activities, site observations, or observed biological effects support portions of the source-to-impairment pathways in the conceptual diagram for nutrients (Figure 3). This diagram and some of the other information also may be useful in Step 3: Evaluate Data from the Case.

The checklist below will help you identify key data and information useful for determining whether to include nutrients among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate excess nutrients as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

Consider listing nutrients as a candidate cause when the following sources and activities, site evidence, and biological effects are present:

Sources and Activities
  • Wastewater treatment plant effluents
  • Industrial effluents
  • Municipal landfills and waste disposal sites
  • Animal feed lots or confined animal feeding operations
  • Construction and development sites
  • Combined stormwater and sanitary sewers
  • Agricultural and irrigation runoff
  • Runoff from impervious surfaces associated with urban or other developed areas
  • Pasture and rangeland runoff
  • Septic systems
  • Atmospheric deposition
  • Landscaping runoff, such as from residential lawns, golf courses, and athletic fields
Site Evidence
  • Proliferation of filamentous algae or algal mats
  • Phytoplankton blooms (i.e., green water)
  • Abundant macrophytes
Biological Effects
  • Alteration of algal assemblages (i.e., phytoplankton or periphyton)
  • Alteration of invertebrate assemblages (i.e., zooplankton or benthic macroinvertebrates)
  • Fish kills

Consider these commonly associated candidate causes when listing nutrients as a candidate cause:

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