Step 4: Classify Water Bodies  into Groups

To classify water bodies for stressor-response analysis, you identify groups of water bodies that have similar stressor-response relationships. Relationships estimated within the groups can be more accurate and precise than relationships estimated using the full data set. Classifying water bodies into distinct groups also provides the means to control for the effects of other variables (i.e., candidate classification variables) on the stressor-response relationship of interest while still allowing for relatively simple models that simulate the response as a function of stressor levels. For example, the coplot of hypoxic extent versus chlorophyll a concentration indicates that the stratification status of a lake strongly influences the slope of the relationships between chlorophyll a and hypoxic extent. This finding suggests that lakes should be classified by stratification status prior to stressor-response modeling. In this case, classification controls for the effect of the stratification and improves the precision of the resulting stressor-response relationships (see Figure 4).

Scatterplots comparing the relationship between hypoxia and chlorophyll a in stratified and unstratified lakes.

Figure 4. Classifying lakes by the strength of stratification reveals a strong relationship between hypoxic extent and chlorophyll a in stratified lakes. The same relationship is much weaker in unstratified lakes.

Statistical approaches are available to automatically screen a large number of variables and identify a combination of classification variables that maximizes the precision of the stressor-response relationship of interest (e.g., TREED regression) (Yuan and Pollard 2014, Yuan and Pollard 2015a).

Case Studies

Yaquina Estuary, OR

  • Yaquina Estuary was classified into two zones:
    • Zone 1—the lower estuary—is more influenced by the ocean
    • Zone 2—the upper estuary—is more influenced by the watershed and point source inputs

Pensacola Bay

  • Pensacola Bay includes three subbays
  • Salinity gradient from Escambia River seaward to Pensacola Pass
  • Nutrient concentrations decrease along salinity gradient

Coastal Bays in MD and VA

  • The bays’ ocean inlets are located at the north and south ends of Assateague Island
  • The bays function as shallow lagoons that do not stratify and have low flushing rates

Barnegat Bay-Little Egg Harbor

  • The estuary is divided into northern and southern segments
  • The northern segment receives greater nutrient inputs
  • The northern segment has a more developed watershed compared to the southern segment

Yaquina Estuary

  • Classified into upper and lower estuary zones
  • Sub-classified into wet and dry seasons

San Francisco Bay

  • Divided into two estuaries: North Bay and South Bay
  • North Bay is a tidal estuary
  • South Bay is a marine lagoon

Nutrients in Neuse River Estuary

  • Split into three segments: upper tidal river, middle segment, and lower estuary
  • Middle and lower segments are naturally divided into two segments at the bend
  • River processes dominate the middle segment
  • Pamlico Sound dominates the lower estuary

Nutrients in Chesapeake Bay

  • Bay is divided into upper, middle, and lower bays
  • Classification based on salinity, turbidity, and other factors

Nutrients in Delaware Estuary

  • Estuary is divided into five regions based on location, nutrient inputs, and turbidity
  • Regions are upper tidal river, urban river, upper bay, mid-bay, and lower bay

Nutrients in Narragansett Bay

  • The bay is divided into the upper and lower bays based on salinity, turbidity, and nutrient inputs

Nutrient Effects in CA Streams

  • A gradient in algal abundance and nutrient concentrations was seen in varying land use

Virginia Freshwater Nutrient Criteria

  • Classify natural lakes and constructed impoundments separately
  • Classify constructed impoundments based on types of fisheries supported and morphometric features

Proposed Criteria for Tampa Bay

  • The mainstem of the bay was divided into four segments
  • Nutrient sources, freshwater inflow, and tides were factors considered in the classification

St. Louis Bay, MS

  • Five spatial strata were identified based on classification process
  • Strata were used as the spatial framework for the analysis

Wisconsin Lake Phosphorus Criteria

  • Examined historical TP data from STORET for Omernik’s 21 subregions and Lillie’s three regions
  • STORET data was insufficient for the 21 subregions
  • Lillie et al.’s (1993) three phosphorus regions were determined as three distinct regions
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