Abstract
Multiple forms of environmental change and anthropogenic pressure co-occur in coastal marine ecosystems. These external forces affect ecosystem structure, functioning, and, eventually, services to humans. Studies that include more than 2 simultaneous stressors are still needed to understand potential interactions among multiple stressors. We evaluated single and interactive effects of density reduction of Zostera marina L. (a habitat-forming species), shading, and sediment nutrient enrichment on the response of Z. marina and its associated epifauna over 10 wk. Shading had the greatest effect on reducing the eelgrass relative leaf elongation rate (RLE), non-structural carbohydrate reserves, and eelgrass shoot density. A reduced eelgrass density sustained higher epifaunal densities and increased the eelgrass RLE. Sediment nutrient enrichment increased eelgrass shoot density but decreased epifaunal richness, diversity, and total abundance. Our disturbance and pair of stressors differed in their influence on diversity measures, but all affected assemblage structure. Most of the changes to the epifaunal assemblage and diversity likely occurred due to altered habitat availability and epiphytic algae load. We observed additive, antagonistic, and negatively synergistic interactions among our treatments, while most of the cumulative effects showed dominance by one stressor over another. Our results highlight the importance of field experiments that are based on multiple disturbances and stressors to determine their interaction type on communities.
Highlights
Seagrasses, which are habitat-forming species, provide a complex habitat that offers several roles, such as reducing water movement, providing shelter from predation, stabilizing sediments, trapping pathogens and sediments, and being important primary producers (Orth et al 1984, Hemminga & Duarte 2000, Herkul & Kotta 2009, Lamb et al 2017)
Shading treatment reduced the number of Z. marina shoots in plots by 14% in the absence of enrichment (N−S+, Fig. 2b; marginally significant, Tukey HSD p = 0.0574), while enrichment increased Z. marina density by 18% in the absence of shading (N+ S−, Fig. 2b; Tukey HSD, p = 0.0106)
The combined effect (N+ S+) was dominated by the effect of shading, as the response size was statistically comparable to the effect of shading alone (N−S+, Fig. 2b; Tukey HSD, p = 0.7694)
Summary
Seagrasses, which are habitat-forming species (or foundation species, sensu Dayton 1972; ecosystem engineers, sensu Jones et al 1994), provide a complex habitat that offers several roles, such as reducing water movement, providing shelter from predation, stabilizing sediments, trapping pathogens and sediments, and being important primary producers (Orth et al 1984, Hemminga & Duarte 2000, Herkul & Kotta 2009, Lamb et al 2017). Their presence provides habitat and protection and food for Publisher: Inter-Research · www.int-res.com. A threshold of habitat loss may exist that, once crossed, leads to a negative effect on epifaunal communities (Reed & Hovel 2006); the existence of such a limit would suggest that a decrease in shoot density may represent one of the foremost signs that the structure and, eventually, the functioning of the entire seagrass bed will be greatly affected
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