Food webs Thematic Assessment

Pressures affecting marine food webs

The main pressures affecting marine food webs are the extraction of wild species and exploitation of living resources, physical disturbance, input of nutrients, non-indigenous species, underwater noise and global warming. The main pressures alter the structure and dynamics of food webs and can have cascading effects through the overall marine food web.

Extraction of wild species and exploitation of living resources [Extraction of living resources]:
Fishing pressure exerts a direct impact on food web functioning by extracting commercial and non-commercial species from the sea, causing individual mortality or injury. Extraction through unsustainable fishing leads to changes in balance across food webs by altering predator-prey interactions and, ultimately, the resilience of the ecosystems. The removal of predatory fish can cause a disproportionate increase in forage fish (plankton feeders), which in turn may deplete the biomass of some zooplankton life forms, leading to imbalances in plankton communities. Fishing not only impacts directly target and by-catch species, but the effects of unsustainable fishing can propagate down through the food web, restructuring the entire ecosystem. The extent to which a disturbance is diminished as it propagates through a food web varies widely between ecosystems, and there is no formal theory as to why this should be so (Heath et al., 2014). Additional effects involve food web processes related to prey availability, competition for food resources, utilization of discards by some species, and / or parasitism. Deterioration in the condition of some fish species due to complex biological and chemical interactions has also been observed. Nevertheless, the intrinsic complexity of food webs makes it particularly difficult to quantitatively discern the effects of fisheries, climate change and environmentally driven variables on ecosystem functioning, even when categorized by low, medium or high fisheries pressure. Ecologically sustainable fishing should not impact resilience. In fact, it should have the opposite effect of mainly removing individuals from abundant stocks, thereby limiting some of the cycling that may otherwise occur in natural populations.

Physical disturbance to seabed (temporary or reversible) [Physical]:
This pressure is strongly related to the extraction of wild species and exploitation of living resources, since one of the main human pressures with a direct impact on the seafloor is fishing. Fishing gears such as trawls and dredges can greatly alter the seabed and exert impacts on the benthic communities and upper trophic levels that feed on small organisms dwelling in or on the seafloor. The magnitude and extent of the impact depends on the gear type and the persistence of the impact. In the OSPAR Maritime Area, bottom trawling is among the main pressures on the seabed. Physical disturbance, no matter the cause, can impact pelagic habitats through changes in stratification / mixing processes that affect primary productivity and trophic relations in the plankton / pelagic compartments (as in the case of offshore wind farms). The extraction of minerals (rock, gravel, sand) may also cause alterations of seabed topography, changes of sediment composition and the removal of organisms. This may lead to shifts in the spatial distribution of species and changes in predator-prey interactions, altering bentho-pelagic coupling and the overall functioning of bentho-demersal ecosystems.

Input of nutrients - diffuse sources, point sources, atmospheric deposition [Substances, litter and energy]:
Classical food web theory suggests that nutrients affect the food web from the bottom up, with top-down effects, through predation, controlling the biomass at each trophic level (Odum, 1969). Therefore, an enhanced input of nutrients may trigger a bottom-up cascade effect by increasing primary production and consequently phytoplankton biomasses and / or changes in their associated diversity, which will impact the upper trophic levels. However, due to the highly complex nature of food web processes it is difficult to demonstrate consistent biological changes based on cause-effect relationships, for example increased nutrient availability leading to increasing biomass of zooplankton and planktivorous fish. Nevertheless, Capuzzo et al. (2018) have demonstrated that concurrent declines in time-series data for primary production, the biomass of copepods and an index of fish recruitment in the North Sea during the 1990s correlated with total oxidized nitrogen in riverine inputs from the Rhine. Through food web modelling of the European Seas, Piroddi et al. (2021) found that planned management measures to reduce nutrients further would have minimal effects on the pelagic offshore marine food web. There may also be consequences for seafloor food webs through increasing sedimentation rates in organic material due to sinking from enhanced plankton production. This ‘marine snow’ can cause increasing oxygen consumption through microbial processes and remineralization in deeper water layers, leading to large areas of oxygen deficiency. Oxygen depletion may also affect the remineralisation capacity of benthic fauna, as well as the distribution and reproduction of higher trophic level organisms.

Input or spread of non-indigenous species [Biological]:
The introduction of non-indigenous species can impact food webs in different ways, for example through the alteration of nutrient concentration and availability, alteration of habitat structure (for example, structural changes of blue mussel beds into mixed beds caused by the Pacific oyster), and competition with native species for food resources and space. The appearance, and especially the proliferation, of a non-indigenous species may cause a cascading effect: high abundances of a non-indigenous species may lead to over-predation of specific prey which in turn could affect other species that feed on the same resource, triggering changes in predator-prey interactions (diet shifts, increased competition–diet overlap–with other species). These cascading effects may change the spatio-temporal dynamics of native species and, ultimately, lead to lower trophic levels and primary productivity, altering the overall functioning of food webs. 

Input of anthropogenic sound (impulsive, continuous) [Substances, litter and energy]:
Many marine mammals and other species rely on their hearing for survival. They use sound to communicate, avoid predation, locate prey or navigate. The noise pollution produced by offshore infrastructure, transport and shipping, ferries, cruise ships and recreational vessels or military activities may alter the behaviour of marine mammals, causing changes in their distribution and migration which in turn may trigger top-down effects by releasing predation over lower trophic levels. Underwater noise may also alter breeding habits and cause injuries or even death to many marine organisms. Although little is known about the direct impact on food webs, some studies point towards a drop in commercial catches due to noise, with larger fish vacating disturbed areas, which can lead to top-down effects (removal of top predators can lead to unwanted abundances at lower trophic levels). The acoustic effects may also increase by-catches, with a consequent impact on food webs through the removal of important compartments of the ecosystem. Underwater noise from commercial shipping is considered one of the most pervasive noise sources. Underwater distribution and noise effects are still unknown; however, impacts have been observed at all trophic levels, from invertebrates to fish, marine mammals, and diving seabirds (Machado et al., 2021).