One human activity can cause many different pressures, and each of these pressures can affect organisms in various ways. The effects can also be hierarchically dependent. For example, the input of chemical substances can lead to reduced available energy of a species due to the energy exerted in combating the chemical. This can lead to reduced energy reserves for reproduction, resulting in negative population effects. Such cascading effects can also result in changes in community composition and biodiversity.
The Baltic Sea impact index uses sensitivity scores based on a regional scale expert survey in order to cover a broad range of topics in a similar way and makes use of existing expertise on the different ways in which pressures may impact the environment. The results can be further validated by a review of selected linkages, available in the literature.
Examples on how such pathways can be outlined systematically using a literature analysis tool are given below. The examples are shown for selected pressures affecting seagrasses and blue mussels, which are keystone species providing habitat for a huge number of other species which interact and are also dependent on one another.
Sea grasses
Major threats to seagrass result from nutrient inputs and habitat loss, the majority of which are from land such as from the oversupply of fertilisers or improperly treated waste water. The increased nutrient levels favour phytoplankton and epiphytes growing on seagrasses, leading to overgrowth and shading and finally to a reduced biomass of seagrass. This effect can be exacerbated by increased current velocities, caused for example by construction activities: snails, normally grazing on seagrass for epiphytes and thus, mitigating the overgrowth effect, are washed away and disappear. Dredging activities bury seagrass and consequently have a direct impact. Additionally, re-suspension of sediments reduces light availability, leading to decreased photosynthesis and decreased growth. Some antifouling additives from ship coating reduces the photosynthetic efficiency of seagrass. Herbicides from agriculture may also affect seagrass and cause similar effects. Increased water temperatures caused by climate change not only affect growth and survival of seagrass but may also favour the spreading of pathogens, such as the potentially epidemic wasting disease which has been responsible for major seagrass declines in the past. Additional important pressures affecting seagrass meadows are for example oxygen depletion and increased sulphide concentrations, direct and indirect effects of fisheries, and acidification (Figure B.6.1.1).
Figure B.6.1.1. Effects of selected human activities on seagrass meadows. Based on systematic literature review using the LiACAT tool (HELCOM 2016h).
Blue mussels
Blue mussels are sensitive to heavy metals and other pollution, since they are filter feeders and accumulate metals directly. Sources of contaminants are industries, land-based activities, air deposition, and activities at sea, such as harbours, shipping, industry, and oil spills. The defence mechanisms that are induced in the mussels are energetically costly for them, and alter heart rate and respiration. Additionally, physical condition is impaired, growth is reduced and mortality increases. The magnitude of these effects is dependent on environmental factors such as salinity, temperature and oxygen conditions. Changes in water temperature can be caused by local industrial heat sources or by climate change. In combination with acidification, effects on early development stages and on shell thickness have been observed. Moreover, shell growth and mortality are negatively affected by the interactive effects of reduced salinity and increased temperature. The dredging effects caused by fisheries activities may lead to decline of blue mussel by removal of species and abrasion of the seabed. The invasive species Crassostrea gigas is considered to compete with blue mussels and may alter the effects of anthropogenic pressures due to different tolerance levels towards the pressures (Figure B.6.1.2).
Figure B.6.1.2. Effects of selected human activities on blue mussels to show the linkage framework. Based on systematic literature review using the LiACAT tool (HELCOM 2016h).
