Stressors and feedbacks in temperate seagrass ecosystems
Seagrasses are rhizomatous flowering plants that cover extensive areas in temperate to tropical coastal areas. Their meadows form the basis for some of the most productive ecosystems on earth and harbor a high biodiversity of marine animal life. In the last century, seagrass ecosystems have suffered large-scale losses worldwide, often characterized by sudden mass mortality of the beds. Despite conservation and restoration efforts and increasing public awareness of the values of seagrasses, this decline is at present still continuing. My thesis focused on determining causes for sudden collapses of seagrass systems, seagrass decline in general, and on identifying predictive tools for seagrass conservation and restoration.
Seagrass loss has typically been ascribed to multiple environmental stressors like for instance eutrophication, disease, salinity changes and toxicity events. I therefore started by investigating the importance of different stressors on presence of the seagrasses Zostera marina and Zostera noltii. I indeed found that many different stressors influenced these seagrasses. However, I also found that presence or absence of these species could generally be explained by measuring only two stressors. This is because many of the stressors are strongly correlated (e.g., surface water turbidity is strongly linked to total nitrogen content). Therefore, to predict seagrass habitat suitability, you don’t need to measure all possible stressors influencing seagrasses; you only need to include the most important ones!
Although (combined) effects of multiple stressors in seagrasses may explain global seagrass decline, it does not explain why these ecosystems often suddenly collapse without clear warning. The second part of my thesis focused on the importance of ecosystem engineering by seagrasses in explaining sudden collapses. I found that seagrasses are able to improve their own habitat. For instance, they reduce hydrodynamics (currents & waves) and stabilize sediments. This in turn leads to clearer water (less suspended sediment), and therefore better growing conditions. However, this self-facilitation mechanism only works if sufficient seagrass is present in the first place! In such systems, a seagrass meadow can suddenly collapse if a disturbance (e.g., disease, storm, or human interference) causes seagrass density to decline below a certain threshold: seagrass is not able to sufficiently improve its own conditions anymore, causing a “domino effect”. After a crash, natural recovery or restoration of the meadow is very hard, because growing conditions are very unfavorable for seagrasses in their own absence.
Overall, I concluded that, although multiple environmental stressors threaten seagrasses, many of these factors are in general strongly related and that habitat suitability can therefore often be estimated by measuring only one or two variables. Secondly, I found that feedback mechanisms are often important in seagrass ecosystems. In these ecosystems, recovery of a collapsed system may be very difficult. In such cases, conservation should focus on identifying indicators to monitor ecosystem resilience and aim at preventing the system from collapsing in the first place.
You can download my PhD thesis as a pdf here!