The purpose of this chapter is to give an explanation of the different stressors and their effects of the marine ecosystem and their interactions.
As we previously stated a stressor is  any stimulus producing mental or physical stress in an organism (Collins English Dictionary). We can group marine stressors in two main categories: a group that acts globally such as increased temperature, ocean deoxygenation (the global trend of decreasing oxygen as a result of ocean warming and increasing stratification) and ocean acidification; and those that act at a local to regional level but occur globally, such as overfishing which is responsible of species loss and population collapse due to the chain effect it has on all the food chain, pollution and hypoxia \citep{Portner_2010}.
The driver of all the global stressors is mainly caused by the excessive release of CO2 in the atmosphere. Although other gasses, like methane (CH)and nitrous oxide (N2O) have a great negative impact on the atmosphere, their effect will respectively last a decade (CH4) and about a century (N2O) \citep[see][]{Intergovernmental_Panel_on_Climate_Change}, every time CO2 is released, 40% will stay airborne 100 years and 20% will reside for 1000 years, while the final 10% will take 10,000 years to turn over \cite{2014}. The ocean is affected directly from the increase of CO2 in the atmosphere with and estimation that 26 percent of all the carbon released as CO2 is absorbed by the ocean (The global carbon budget 1959–2011 C. Le Quere). 
Ocean Warming. When sunlight reaches the Earth’s surface, the world’s oceans absorb some of this energy and store it as heat which is first captivated on the surface but ultimately some of it is spreads into lower depths. Water has a much higher heat capacity than air, meaning the oceans can absorb larger amounts of heat energy with only a slight increase in temperature \cite{policy} although having a great negative impact on some of the fine balanced ecosystems found in the ocean.  
''Ocean deoxygenation refers to the loss of oxygen from the oceans due to climate change (Keeling et al. 2010). Oxygen plays a key role in structuring marine ecosystems and controls the distribution of essentially all marine organisms (Gilly et al. 2013) ''
Ocean acidification (OA) is primary caused by the atmospheric CO2 absorbed by the ocean \citep{Feely_2010}  affecting the water chemistry. The impacts of OA are not uniform across all species, some phytoplankton, algae and seagrass may also thrive with higher CO2 concentrations in the ocean, as they may increase their photosynthetic and growth rates although. However, a more acidic environment will harm other marine species such as molluscs, corals and some varieties of plankton. The shells and skeletons of these animals may become less dense or strong. In the case of coral reefs this may make them more vulnerable to dissolution storm damage and slow the recovery rate \citep{Haigh_2015}.
As a local but globally spread stressor, Pollution covers a broad range of categories but the biggest source of this stressor is non-point source pollution which occurs as a result of runoff from urbanised areas (cars, trucks, septic tanks, farms, livestock ranches, and timber harvest) adding chemicals, heavy metals, oil and other pollutants which greatly affect the water quality. Plastic has also a important impact on the ocean directly affecting all the biota in the ocean \citep{Koelmans_2014}.
Hipoxia The increasing frequency of blooms of nontoxic micro-algae and nuisance macro-algae is harmful because it dramatically reduces sunlight penetration in the water column. Large and long blooms can also draw a substantial amount of oxygen from the water column at night during respiration. Even though these events can occur seasonally, increasing water temperature and OA can change their frequency and intensity favouring the expansion of dead zones \citep{Sherman_2017}

Interaction between stressors

. While interactions between stressor are highly complex to measure the prevailing view is that in a situation where three or more stressors (for example Ocean warming, OA, Overfishing) are affecting the ecosystem, synergies are expected to be the most prevalent interaction type (Paine et al. 1998). On other hand some interaction might also experience an antagonistic (reduced stress) or  additive (no additional stress) effects. When a system is affected by multiple stressors it also diminishes its resilience and recovery giving space increasing vulnerability towards disturbances that would generally not have any affect on an healthy ecosystem (Beaugrand et al. 2010).