Appendix 6.9 Description of “stressors” and how they fit into the ocean accounts framework.

Sea Level Rise is considered a change in condition in the Condition Accounts. It is driven primarily by thermal expansion of seawater and, to a lesser degree, melting of ice in the polar oceans. It could cause changes in ecosystem extent (e.g., coastal settlements, infrastructure, beaches, wetlands, mangroves), changes in services (e.g., coastal protection, filtration, amenity value) if it occurs over extended periods (Nichols and Cazenave, 2010).

Ocean Warming is also a change in condition. It is an increase in sea surface temperature (SST) due to increased accumulation of greenhouse gasses (CO₂, CH₄, N₂O) in the atmosphere, thereby trapping in heat and warming the land and surface oceans. Ocean warming can impact other conditions, such as organism metabolic rates and resulting oxygen availability, storm frequency, mean sea level (see Sea level rise above), rainfall patterns, and most notably long-term changes in thermohaline circulation. Such conditions are usually tracked in more detail in other frameworks (e.g., IPCC; Clarke et al., 2013).

Eutrophication (i.e., elevated organic matter concentration) is a condition characterised by excess concentrations of particulate and dissolved organic matter. Abnormally high concentrations of organic matter are created by the rapid bloom and die-off of plankton populations, generally stimulated by excess dissolved inorganic nutrients. These nutrients may come from terrestrial runoff events (flows from the economy to the environment), deep-water upwelling events, or the over-retention of locally produced nutrients due a reduction in seawater mixing (e.g., stagnation) (Bell, 1992; Menzel et al., 2013).

Ocean Acidification (i.e., low pH, reduced calcification, and dissolution of calcium carbonate) is a condition caused by increased sequestration of atmospheric CO₂ into the surface oceans and is of primary concern in environments where a large part of the benthic sessile community conducts calcification (e.g., coral reefs). Acidification may also happen in coastal locations (Coastal Acidification) due to a biological reduction in seawater pH due to eutrophication (remineralization of dead plankton) (Kleypas et al. 2001; Hofmann et al., 2011).

Hypoxia/Anoxia (i.e., low to no dissolved oxygen) is a condition that leads to widespread die off of the local fauna and can be caused by differing processes such as eutrophication, changes in the physical oceanography, seawater warming, or drought (Chan et al., 2008; Barbier et al., 2011).

Pollutants (residuals, plastics, nutrients, sediments, solid waste, marine litter etc.) are both flows from the economy to the environment and conditions. Measures of the flows (loads, in tonnes) provide insight into the reasons for changes in condition. Condition measures are usually in terms of concentrations in the water, substrate and biota. To provide useful condition indicators, the measures should be indexed to a reference condition, such as “natural” or “suitable for wildlife”.

Overfishing is an assessment of the degradation of ocean assets. It is caused by more fish being harvested than the natural regeneration rate. Fish stocks by species are recorded in the Asset Accounts as “individual environmental assets”. Fish harvesting is recorded as a flow from the environment to the economy and as a provisioning service. Fish harvest is indicated in the Ocean Asset Accounts as a reason for reduction in stocks. Natural regeneration is recorded as a natural increase in stocks. If reductions exceed regeneration or replenishment, an assessment would need to be made if this is “overfishing” by comparison to an established Maximum Sustainable Yield (MSY) and other explanatory factors (e.g., change in temperature, fishing effort, increase in predator populations) (Steneck et al., 2002; Cesar et al., 2003; Halpern et al., 2006) .

Habitat Loss results from conversion of an ecosystem from one type to another. This is recorded as changes in extent in the Extent Accounts. Loss of habitat may occur in response to the 7 stressors above but may also occur due to other reasons (e.g., coastal development or disease). It may also be caused by extreme changes in condition (e.g., disturbance by humans or unsuitable levels of pollutants). This would be recorded in the Condition Accounts if it occurs over several accounting periods. Habitat locations are often determined using models based on conditions that are typical for a specific species. As such, “habitats” are generally much more localized than ecosystem types. Changes in extent may result in further changes in conditions (e.g., reduction in biodiversity) and ecosystem services (e.g., maintenance of chemical conditions of water) (Barbier et al., 2011).