Coastal ecosystems are exposed to a plethora of direct and indirect anthropogenic stressors such as climate warming, eutrophication, metal pollution, hypoxia, pH and salinity changes, and overfishing (Halpern et al., 2007; Crain et al., 2008; Griffen et al., 2016; Gissi et al., 2021). These stressors are inducing serious and irreversible changes in marine and coastal food webs (Carrier-Belleau et al., 2021), and they do not act in isolation but instead simultaneously (Griffen et al., 2016). It has been suggested that by 2050 about 90% of the global ocean will be impacted by exposure to multiple stressors (Henson et al., 2017). Interactions between stressors trigger responses in species and communities that are often different from the effects of the individual stressors (Crain et al., 2008; Jackson et al., 2016).

To understand the changes in coastal ecosystems and to improve marine and coastal management, a better knowledge of stressor interactions is required (Gladstone-Gallagher et al., 2023; Gissi et al., 2021). The interactive effect of multiple stressors could be the sum of the individual responses, more than that, or less. When the combined effect of multiple stressors is equal to their expected cumulative sum, such an effect is called additive. When this is not the case, it is termed multiplicative or non-additive (both used interchangeably) (Pirotta et al., 2022). A non-additive effect could, in turn, be synergistic or antagonistic. When the net effect of interactions is greater than the sum of individual stress responses, the response is called synergistic (Crain et al., 2008; Jackson et al., 2016). On the contrary, in an antagonistic interaction, the effect intensity of the combined response is less than that of the cumulative single-stressor effects.

Occurrences of additive and non-additive interactions have been reported in coastal ecosystems (Crain et al., 2008; Villar-Argaiz et al., 2018). For example, synergistic responses to elevated nutrient concentrations and metal loadings have been found in diverse plankton groups (Bundy et al., 2003; Wiegner et al., 2003; Simboura et al., 2016; Abbate et al., 2017). Negative synergistic effects have been reported in benthic organisms (such as crabs and bivalves) on exposure to ocean acidification (OA), salinity, temperature, and metal pollution (Stueckle, 2008; Miller et al., 2014; Brooks and Crowe, 2019). The combined effects of heat stress and OA have been far more adverse than their individual effects in a variety of mussel species (Vihtakari et al., 2013; Rodrigues et al., 2015). Likewise, the adverse effects of overfishing can be intensified by climate warming in top predatory fish species (Ainley and Blight, 2009). Stressor interactions also lead to antagonistic responses in coastal ecosystems. For example, it has been shown that elevated CO₂ resulted in higher chlorophyll biomass under low irradiance, thereby compensating for light stress (Neale et al., 2014; Heiden et al., 2018). Similarly, low pH has been shown to have mitigated the negative effect of high temperature on egg volume in an estuarine fiddler crab (Pardo and Costa, 2021).

In spite of the significant role of stressor interactions in driving ecosystem and community-level responses, investigations of these effects are largely disconnected from the implementation of conservation and management policies for coastal systems (Gladstone-Gallagher et al., 2023; Côté et al., 2016). Over the last decades, research and management focused on eutrophication problems in coastal environments (Smith and Schindler, 2009). The non-additive effects of eutrophication with other stressors such as climate warming, pollutants, hypoxia and changes in salinity have posed a multidimensional problem (Hewitt et al., 2016; Henson et al., 2017; Thrush et al., 2021). First, the intricacy of interactive effects makes it very difficult to elucidate the net response of an ecosystem in ever-changing environments (Carrier-Belleau et al., 2021). Secondly, the occurrence of several stressors triggers differential responses across organisms. Thirdly, critical stressor combinations vary between taxonomic groups.

In this study, we address the aforementioned research gaps. With a focus on high-latitude coastal systems, we systematically review the existing studies on the interactive effects of multiple stressors in coastal habitats to answer the following important questions: 1) What are the critical stressors for triggering non-additive responses in coastal ecosystems? 2) What are the main stressor combinations for different taxonomic groups with respect to inducing negative synergistic effects? 3) What are the research gaps in multi-stressor studies? From our findings, we provide valuable insights to scientists and stakeholders with respect to the advancement of multi-stressor research and for management of coastal ecosystems.

A systematic literature search for scholarly articles relating to the topic was performed using Clarivate’s Web of Science (WoS) advanced search and Google Scholar with similar functions. The search included a topical search for multiple stressors, compound, cumulative or interactive effects at the coast or on the shelf, classified as an article or review. We implemented the following search query: “TS=(((“multipl* stress*”) OR (“compound* effect”) OR (“cumulat* effect”) OR (“interact* effect”)) AND (coast* or shelves or shelf)) AND DT=(“ARTICLE” OR “REVIEW”)”

Abstracts of all obtained records were prescreened independently by two researchers for false positives that should be discarded as not relating to the topic, e.g. articles relating to management and policy, society and socio-ecological systems, single stressors, freshwater systems, marshes or seismology. The review papers contained in the remaining records were screened for references to primary relevant literature that was not captured by the automated search. Citations within review papers that were identified as relevant after screening their abstract were included in the subsequent full-text analysis. All remaining and manually added full texts were distributed for detailed evaluation among the author team. We employed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses [PRISMA, Moher et al. (2009)] approach for our analysis. The systematic search, based on queries and keywords, on WoS and Google Scholar yielded 814 papers that investigated the interactive effects of multiple stressors in marine ecosystems. All of these papers were screened, first by an automated script and then manually, to select the studies that focused only on high-latitude coastal ecosystems, which reduced the number of papers from 814 to 400 ( Figure 1 ). Out of these, a few were review or synthesis papers which were discarded from the analysis, and the remaining were distributed amongst the co-authors to review and fill up the “Summary Table” ( Table 1 ). In total, we could identify 198 studies in which non-additive or additive effects were reported ( Figure 1 ). All further analyses were performed based on the information provided in this table.

To quantify the magnitude of interactive effects (synergistic = SYN, additive = ADD, and antagonistic = ANT) for stressor combinations, we implemented a fuzzy coding/scoring method where scores between 1 to 3 were assigned for the reported responses. For example, if a study reports only a synergistic effect for a given stressor combination, a score of 3 is assigned for SYN and a score of 0 for ANT and ADD. If two types of effects (e.g. SYN and ANT or ADD and SYN) are reported then a score of 1.5 is given to each and 0 to the third effect which is missing. And, if all three effects (SYN, ANT, and ADD) could be identified then a score of 1 is assigned to all. For a given stressor pair, the sum of all three effects is always 3 (SYN + ANT + ADD = 3). If there are more than two stressor combinations, then the same procedure is followed for the respective stressor pairs resulting from that particular combination (e.g. 4 stressor combinations yield 6 stressor pairs). The final score for a given effect (SYN or ADD or ANT) is then calculated by adding up their individual scores identified from different stressor combinations. The detailed schematic of this scheme is illustrated in Figure 2 .

Interactive effects are reported at different levels of the coastal ecosystem; the majority (n=109, 55%) of studies focused on the species or organism level, 36% (n=71) on the community level, and 9% (n=18) on the entire ecosystem. The types of effects reported can be classified as synergistic, antagonistic, or additive. Most of the studies either reported additive (n = 62) or synergistic (n = 68) effects, and a few (n=15) reported both ( Figure 3 ). These findings are indicative of the intensification or compounding of responses to multi-stressor exposure in coastal ecosystems. Whereas ANT effects are identified in only 20 studies (10% of the total). A handful of studies (n = 5) reported both ADD and ANT effects. Likewise, only 12 papers identified both SYN and ANT effects. Lastly, a few (n = 16) reported all three effects.

Next, we identified the most investigated stressors that affect various taxonomic groups and communities in coastal ecosystems. The most frequent individual stressors in a multi-stressor constellation are temperature (Temp, 25%, n=50), followed by nutrient loading (Nut, 17%, n=33) and toxic metals/pollution (Metal) and ocean acidification (OA, together 28%, n=56) ( Figure 4 ). While high turbidity and salinity stress are moderately studied (both 11%), hypoxia (DO) and physical/mechanical disturbances (Dist) are the least studied stressors. Quantification of the interactive effects (SYN, ANT, and ADD) for the stressor combinations of the most reported stressors is shown in Figure 5 . The combinations of acidification– eutrophication (OA and Nut) and metal pollution–eutrophication (Metal and Nut) have mostly been reported as synergistic (with >50% score), whereas acidification–metal pollution (OA and Metal) has been reported mostly as additive (∼ 70% score). The Warming–acidification (Temp and OA) combination is expressed equally as additive, synergistic and antagonistic.

The above results provide aggregated information about the critical stressors in coastal ecosystems. However, the relevance of these stressors can vary depending on organism type, trophic level, and trophic structure. Therefore, we identified the key stressors that elicit interactive effects at the species and community levels. Temperature is, by far, the most critical stressor at the species level. One-third of the studies (33%) investigating interactive effects in individual species have identified temperature as the most prominent stressor ( Figure 6 ). However, this is not the case at the community level, where warming, nutrient pollution and metal contamination emerge as equally dominant stressors, followed by high turbidity, low pH, and salinity stress (all above 10%, Figure 6 ). While, hypoxia remains heavily understudied (<4%).

At the species level, the combination of metal and nutrient pollution predominantly drives synergistic effects, whereas warming and eutrophication equally trigger both SYN and ADD responses. ( Figure 7A ). In contrast, OA combined with other stressors appears to invoke differential responses depending on species type, as evident by the lack of a dominant effect in stressor combinations involving OA ( Figure 7A ). At the community level, climate warming and ocean acidification have the highest likelihood of generating synergistic responses ( Figure 7B ).

Subsequently, we identified the most studied organisms and their respective traits for the major stressor combinations. Bivalves and phytoplankton are the most studied taxonomic groups, followed by seagrass and fish ( Figure 8 ). This difference could be partially attributed to research constraints arising from the complexity of studied organisms or systems. At higher body size the fraction of manipulative experiments decreases. For smaller organisms, such as phytoplankton, zooplankton, and bivalves, more than 50% of studies are performed in controlled laboratory setups, whereas this fraction falls below 50% for fish and decapods and below 20% for seagrass ( Figure 9 ). Larger organisms are preferably studied in their natural and quasi-natural habitats. Likewise, in-situ experiments are preferred for studying ecosystem-level effects of multiple stressors.

Metal pollution, eutrophication, and climate warming seem to be the most dominant stressor combination for phytoplankton, strongly affecting photosynthesis and growth dynamics in algal species ( Figures 8 , 10 ). For bivalves, OA and Temp are the most critical stressors in driving non-additive effects ( Figures 8 , 10 ). A wide range of traits are affected by the combination of OA and Temp stressors in bivalves, including fitness, defense, abundance, and metabolism ( Figure 8 ). Likewise in fish species, Temp, OA and Sal stressors affect a broad range of traits. For seagrass, the combinations of eutrophication, warming, and salinity stress are equally critical, affecting their growth and metabolism ( Figure 8 ).

As bivalves and phytoplankton are the most studied taxonomic groups, we investigated which combinations of stressors are most likely to generate synergistic responses. We identified them by a fuzzy coding approach, where stressor combinations were given scores for synergism. For phytoplankton, the metal-nutrient (Metal and Nut) combination got the highest score ( Figure 8 ), as exposure to nutrient and metal pollution typically instigates synergistic responses at the physiological level in autotrophs. Likewise, the temperature-acidification (Temp and OA) stressor pair got the highest score for bivalves ( Figure 8 ), indicating that the combination of warming and ocean acidification is likely to synergistically affect their physiological rates.

In our review analysis, we identified climate warming, eutrophication, ocean acidification, and metal pollution, or combinations thereof, as the most critical stressors triggering non-additive responses in coastal ecosystems. Thus, from a management perspective, particular attention should be placed on mitigating these stressors in coastal systems. Phytoplankton and bivalves are the most studied taxonomic groups in multi-stressor experiments and observations. They are, however, sensitive to different stressor combinations. Simultaneous exposure to metal pollution and high nutrient concentrations invokes synergistic responses in phytoplankton, often with negative effects on their physiology. For bivalves, climate warming and ocean acidification appear to be the most critical stressor combination, with adverse effects on physiological and morphological traits. Organisms at different trophic levels, or belonging to different ecosystem components (such as micro- and macrobenthos), are sensitive to different stressor combinations. Consequently, interactive effects could induce radical changes in trophodynamics and food web structure as climate and human-induced changes in coastal ecosystems continue to intensify. For a holistic understanding of the cumulative effects of multiple stressors in coastal ecosystems, however, we suggest that more research focus should be placed on studying community and ecosystem-level responses. This can be achieved by combining in-situ observations (e.g. mesocosm experiments) with modelling approaches. In addition, other relevant taxonomic groups such as zooplankton, fish and benthic polychaetes demand more research.