Guideline 11: Spatial Variability 



Indicator responses to various environmental conditions must be consistent across the monitoring region 

 if that region is treated as a single reporting unit. Locations within the reporting unit that are known to be 

 in similar ecological condition should exhibit similar indicator results. If spatial variability occurs due to 

 regional differences in physiography or habitat, it may be necessary to normalize the indicator across the 

 region, or to divide the reporting area into more homogeneous units. 



Monitoring programs that cover large geographic areas must contend with the inherent spatial variability in 

 the data. In the Louisianian Province, which spans estuaries from Texas to Florida, EMAP-E encountered 

 the full range of expected benthic habitat types (from sand to mud, tidal freshwater to marine, and impacted 

 to pristine) and identified more than 1000 different benthic invertebrate species. The 46 test sites that were 

 used to develop the benthic index were chosen, therefore, not only to represent extreme reference and 

 degraded environmental conditions, but also to cover the expected range of salinity, sediment types, and 

 biogeographical divisions found in the estuaries of the northern Gulf of Mexico (Engle et al. 1 994, Engle and 

 Summers 1999). The sites ranged in salinity regimes from tidal-freshwater (0 ppt) to marine (>35 ppt) and 

 most sites were located in muddy (>80% silt-clay) sediment (Table 3-6). The location of the majority of sites 

 in Louisiana is simply an artifact of the EMAP probability-based sample design (Louisiana has proportionately 

 more estuarine area than the other four gulf states). In this way, we sought to minimize the spatial variability 

 in the benthic index. 



Table 3- 6. Distribution of the number of degraded and undegraded test sites among 

 categories of salinity, sediment types, and states. 



Salinity Number of Number of 



Degraded Sites Undegraded Sites 



Fresh (0 ppt) 8 2 



Brackish (>0-5ppt) 2 3 



Mesohaline (>5-18 ppt) 4 5 



Polyhaline(>1 8-35 ppt) 7 11 



Marine (>35 ppt) 1 3 



Sediment Type 



Mud (>80% Silt-Clay) 17 10 



Mud/Sand (20-80% Silt-Clay) 5 10 



Sand (<20% Silt-Clay) 4 



State 



Florida 4 2 



Alabama 3 3 



Mississippi 2 2 



Louisiana 12 10 



Texas 1 7 



3-19 



