In this example, the assessment question is: What percent of estuarine area is hypoxic/anoxic? Since 

 hypoxia and anoxia are defined as low levels of oxygen and the absence of oxygen, respectively, the 

 relevance of the proposed indicator to the assessment is obvious. It is important to note that, in this evaluation, 

 we are examining the use of DO concentrations only to answer the specific assessment question, not to 

 comment on the eutrophic state of an estuary. This is a much larger issue that requires additional indicators. 



Guideline 2: Relevance to Ecological Function 



It must be demonstrated that the proposed indicator is conceptually linked to the ecological function of 

 concern. A straightforward linl< may require only a brief explanation. If the link is indirect or if the 

 indicator itself is particularly complex, ecological relevance should be clarified with a description, or 

 conceptual model. A conceptual model is recommended, for example, if an indicator is comprised of 

 multiple measurements or if it will contribute to a weighted index. In such cases, the relevance of each 

 component to ecological function and to the index should be described. At a minimum, explanations 

 and models should include the principal stressors that are presumed to impact the indicator, as well as 

 the resulting ecological response. This information should be supported by available environmental, 

 ecological and resource management literature. 



The presence of oxygen is critical to the proper functioning of most ecosystems. Oxygen is needed by 

 aquatic organisms for respiration and by sediment microorganisms for oxidative processes. It also affects 

 chemical processes, including the adsorption or release of pollutants in sediments. Low concentrations are 

 often associated with areas of little mixing and high oxygen consumption (from bacterial decomposition). 



Figure 2-1 presents a conceptual model of oxygen dynamics in an estuarine ecosystem, and how hypoxic 

 conditions form. Oxygen enters the system from the atmosphere or via photosynthesis. Under certain 

 conditions, stratification of the water column may occur, creating two layers. The upper layer contains less 

 dense water (warmer, lower salinity). This segment is in direct contact with the atmosphere, and since it is 

 generally well illuminated, contains living phytoplankton. As a result, the dissolved oxygen concentration is 

 generally high. As plants in this upper layer die, they sink to the bottom where bacterial decomposition 

 occurs. This process uses oxygen. Since there is generally little mixing of water between these two layers, 

 oxygen is not rapidly replenished. 



This may lead to hypoxic or anoxic conditions near the bottom. This problem is intensified by nutrient 

 enrichment commonly caused by anthropogenic activities. High nutrient levels often result in high 

 concentrations of phytoplankton and algae. They eventually die and add to the mass of decomposing 

 organic matter in the bottom layer, hence aggravating the problem of hypoxia. 



2-2 



