Excerpts from Moss et al. (2006) 
Stressors were defined in  as follows: Physical, chemical and biological stressors are major components of the environment that, when changed by human or other activities, can result in degradation to natural resources. Stressors can be:
The definition of stressors is somewhat broad, and identification of stressors within the limits of this definition is still largely based on expert opinion .
Based on excerpts from Scheltinga et al. 2004 :
The stressor terminology was designed to provide a common starting point for the process of selecting
indicators based upon regional situations. Key environmental stressors
are used in the framework rather than specific natural resource management (NRM) issues because stakeholders
do not all use the same language when describing these issues. NRM issues can be defined as causes or
symptoms of natural resource problems (Figure 1), and the terms "pressures", "issues" "problems" or even
"indicators" are often used interchangeably. Therefore, it is not possible to compile a comprehensive
list of NRM issues with specific indicators for each.
Figure 1. This diagram shows that causes or symptoms of natural resource problems and stressors are all commonly used to describe NRM 'issues'. Note also that community-held values influence the choice of indicators through the causes (management actions) and stressors to be addressed by management actions [from 2].
A set of 16 defined stressors were identified in a former Coastal CRC project  and these were subject to expert review at two national workshops, held under the auspices of NHT and NAP. These stressors are listed below. You can follow the hyperlinks to obtain lists of the causes of change to stressors, some of the symptoms (i.e. impacts on the system) that might result from them, some potential indicators to monitor the stressors and links to external information sources.
View new conceptual models illustrating 12 important stressors affecting your waterways: Stressor models are used to show how human activities (i.e. threat models) interact with the natural processes occurring in an estuary (i.e. process models) and impact the ecosystem. This allows pressure and condition indicators to be determined that are specific to a particular stressor and estuary type.
A set of 13 defined stressor models were identified by