Seagrass responses to interacting abiotec stresses

Seagrasses are coastal and marine flowering plants that inhabit thetropical and temperate coastal and marine areas around the globe. They havemany important functions and values, physically, ecologically andeconomically. Physically, seagrass contribute to coastal protection as theyattenuate wave energy and stabilizing sediments. Through this, neighboringecosystem may benefit because water movement is reduced and suspendedsediment to settle, thereby decreasing water turbidity. Moreover, this results in apositive feedback to the seagrass, as it sustains light intensity for plants’photosynthetic activity. Ecologically, seagrass can function as habitats forvarious marine organisms as their feeding, nursery and/ or as their breedingground. This way, seagrass meadows sustain both biodiversity as well ascommercially valuable species. However, regrettably, our seagrass ecosystemsare declining around the globe. Anthropogenic activities are the major cause forthis decline, rather than natural events. Yet, due to climate change, extremeweather events (such as increasing storm frequencies and intensities) areexpected to increase, which may worsen the current condition. We have stillmuch to learn about how seagrass can cope with these environmental stresses,in order to better manage our seagrass ecosystems and to reduce their loss.Hence, we studied how seagrass respond physiologically, morphologically andbiomechanically to a range of environmental stresses: being exposed to low andhigh nutrients, high hydrodynamic and reduced light.In nature, seagrasses encounter a mixture of inorganic and organicnitrogen containing substances with varying bioavailability in lowconcentration. Yet, seagrass research had mainly focused on nitrogen uptake ona single nitrogen substrate at a time. By using a combination of one of 15Nlabeledsubstrate and one 14N-unlabelled background substrate we demonstratedseagrass “constitutive preference” for ammonium uptake over other substrates(i.e., nitrate as the dissolved inorganic nitrogen and urea or glycine as thedissolved organic nitrogen). However, substrate uptake was always independentfrom the background nutrient (which occurred both in above and belowgroundplant parts), implying that there was no down- or up-regulation that favouredone nitrogen source over the other (induced preference). This was probably dueto the low (but realistic) concentration used in the experiment as compared toother studies that showed both “constitutive” and “induced” preferences ofammonium on nitrate uptake. For the dual labeled (15N and 13C) urea andglycine, a strong relationship existed between nitrogen and carbon uptake, butwith deviations from expectations under complete uptake of the molecules. Insummary, we may conclude that at realistically low (ambient) nutrientconcentrations, seagrasses can use inorganic as well as organic nitrogen sourcesChapter 7128and do not differentiate between substrates. In other words, seagrasses arecapable to take up whatever is available (chapter 2), yet the eventualcontribution of different sources in the overall nitrogen uptake may furtherdepend on the relative concentrations of the different sources. Indeed, havingthe capability to take up whatever is available may serve as an advantage forseagrass exposed to low nutrient environment.Apart from the benefit to be able to take up whatever is available in lownutrient environments, seagrass mechanical performance also benefits from lownutrient conditions. That is, under low nutrient conditions, seagrasses are strong(having higher specific-force-to-tear, FTS) as shown for Halophila ovalis(chapter 3), Zostera noltii (chapter 5) and Enhalus acoroides (chapter 4). Incontrast, under high nutrient concentrations, seagrass can easily be broken.Light deprivation is another environmental stress that can cause seagrass toeasily break (chapter 3). Both high nutrient and light deprivation causedseagrass (Halophila ovalis) leaves to be weakened by having lower breakingstress (FTS) even though the absolute breaking force (FMAX-a size-dependentmaterial property) stayed the same. Under nutrient enrichment, however,seagrass Zostera noltii had lower breaking stress (FTS) as well as lower breakingforce (FMAX) This may imply that leaf mechanical resistance may result fromacclimation to any environmental heterogeneity by both morphological andmechanical changes (chapter 3). Seagrass Halodule uninervis however, did notshow any prominent changes in their morphology and mechanical performancedue to environmental changes. Thus, mechanical responses are species-specificwhich was also observed in seagrass Enhalus acoroides and Thalassiahemprichii (chapter 4). Apart from being mechanically species-specific,seagrass also possessed intra-species specificity in terms of their morphologiesand mechanical properties. Leaf and sheath tissues have obvious differences intheir morphology. Similar differences exist for the mechanical properties. Forexample, sheaths of Enhalus acroides exhibit higher extensibility (by being ableto extend more before they break) than their leaves. This is probably due to thesofter meristematic tissues that form the sheaths that still bear elastic cell walls.Yet, in contrast, sheaths of Thalassia hemprichii had a lower breaking forcethan their leaves. Because the sheaths are short and (most of the time) are in thesediment, hence having stronger leaves for Thalassia hemprichii is probablymore necessary than having stronger sheaths.Apart from seagrass mechanical and morphological responses to (single)environmental stresses as mentioned above, in this thesis, we also revealedexciting new findings on the effects of interacting wave and nutrient stress(chapter 5). Wave and high nutrient stress decrease survival of seagrass Zosteranoltii. However, waves independently reduce seagrass length and belowgroundbiomass, whereas high nutrient concentrations reduce seagrass strength andSummary129stiffness. The latter was in agreement with our observations for Halophila ovalis(chapter 3). These specific responses of seagrass Zostera noltii to interactingstress of waves and high nutrient conditions may be expected to induce negativefeedbacks that may eventually result in meadow collapse. For example, adecrease in leaf length and aboveground biomass due to waves can reduce waveattenuation capacity of the bed hence can increase stress to seagrass by a furtherreduction of aboveground biomass and leaf length. This condition, however, canbe exacerbated by an increasing loss by uprooting. In addition, high nutrientstress can result another potential negative feedback for weakening the plantswhich eventually can increase plant losses. Obviously, in combination, thesesnegative feedbacks enforce each other even further, accelerating potentialcollapse.With respect to mechanical properties, some important generalizationsarise from comparing all chapters. There seem to be a general trend where slowgrowingseagrass have a higher leaf resistance (FMAX) and are stronger (i.e.,higher breaking stress, FTS) compared to fast-growing ones. The slow-growingspecies apparently compensate their low leaf turn-overrate by having strongerleaves that allow for a longer leaf life span. To be long-lived, it may be aprerequisite to have strong leaves in order to cope with environmental stresses(abiotic and biotic). Understanding the mechanical properties for living materials such asseagrass tissue, is more complex than obtaining such understanding for mostengineered material. This thesis aimed at increasing our understanding of thecomplexity of seagrass mechanical traits in response to environmental stressesand to contribute to our knowledge on the function and ecological significanceof biomechanical properties.