A trait-based perspective on seagrass ecology: from plant to ecosystem level processes

Abstract

Seagrasses are marine flowering plants that inhabit the coastal area forming important ecosystems due to a number of ecosystem services they provide. However, they are subjected to both global and local impacts, including warming water temperatures and eutrophication, which threaten their survival. Despite the fact that the most diverse seagrass meadows are found in the tropical Indo-Pacific Region, there is less information about tropical species than their temperate counterparts. There are, therefore, knowledge gaps in the response of tropical seagrass meadows to environmental drivers and their links to ecosystem functions and services. In the last three decades, trait-based frameworks (TBFs) have advanced different fields of ecological research through establishing novel links between functional traits, environmental drivers and ecosystem functions. A number of concepts have been proposed in order to answer different ecological questions using a functional trait-based perspective. This field of research has been widely developed in terrestrial plants. However, the use of TBFs in seagrass research is currently in its infancy.

The goal of this dissertation is the incorporation of TBFs into seagrass ecological research, by establishing novel links between seagrass traits, environmental drivers and ecosystem functions and services. The study site chosen for this work was Unguja Island (Zanzibar Archipelago, Tanzania). Unguja Island is located in the tropical Indo-Pacific region, and is considered one of the hotspots of seagrass biodiversity worldwide. The seagrass meadows in Unguja Island are subjected to a wide range of conditions, from pristine and oligotrophic to heavily impacted and eutrophic. Due to its high seagrass diversity and the variety of conditions under which seagrass survive, Unguja Island is a perfect laboratory for the study of seagrass communities using a TBF. The research questions selected for this dissertation have the goal of understanding the importance of traits at different organizational levels, from their individual responses to environmental drivers, to the effect of traits on the interspecific competition of seagrass species and, lastly, their effect on ecosystem functioning.

First, to assess the knowledge gaps in seagrass trait-based research, I carried out a systematic review of the seagrass literature. The analysis showed that seagrass trait research has mostly focused on the effect of environmental drivers on traits (65%), whereas links between traits and functions are less common (33%). Despite the richness of trait-based data available, concepts related to TBFs are rare in the seagrass literature (7% of studies). These knowledge gaps in seagrasses indicate ample potential for further research. In order to address these knowledge gaps, I propose a TBF that can help guide future seagrass research.

Secondly, the responses of traits of individual seagrass plants of tropical seagrass species (Halophila stipulacea, Cymodocea serrulata, Thalassia hemprichii and seedlings of Enhalus acoroides) were assessed to two environmental drivers: temperature (global) and nutrient enrichment (local). To achieve this aim, a 1-month experiment under laboratory conditions combining two temperature (maximum ambient temperature and current average temperature) and two nutrient (high and low nitrogen and phosphorus concentrations) treatments was conducted. The results of this experiment showed that trait responses are species-specific, and that temperature was a much more significant driver than nutrient enrichment. In the case of the seedlings of E. acoroides, they rely energetically in the reserves within the seedling and increasing temperature resulted in faster seedling development. T. hemprichii and C. serrulata showed an enhanced morphology, while the contrary was true for H. stipulacea. These results highlight the different effects and strategies that co-inhabiting seagrasses have in response to environmental changes. Thirdly, an experiment was developed in the field to test the effects of light shading and trampling due to the farming of Euchema denticulatum on seagrass meadows, an environmental driver endemic to the tropical region. Areas covered by T. hemprichii, H. stipulacea were selected for the building of seaweed farms for 3 months. Light was reduced in the seaweed farm plots by 75 to 90% by the end of a seaweed growth cycle. The responses of seagrass were, again, species-specific. H. stipulacea, despite its capacity for rapid growth, was significantly affected by the combination of shading and trampling under the seaweed farm treatment, while the climax seagrass species T. hemprichii was unaffected.

Fourthly, to link individual plant traits to seagrass community level processes, I carried out an observational study in Unguja Island. The goal was to understand how seagrass traits linked to light and nutrient competition affected space preemption among seagrass species under different trophic scenarios. Traits determining the functional strategy of the seagrass showed that there was a size gradient in the seagrass species. When tested the effect of the difference in the functional strategy of species pairs, the probability of preemption was highest for the bigger species, increased when their size difference was higher and was not affected by the eutrophication. This indicated that the competitive interactions among seagrass species were asymmetrical, i.e. a species had a negative effect on another species, while the effect was not reciprocal and the driver behind space preemption was determined by traits related to the size of the seagrass plants.

Fifthly, to study the link between seagrass traits and ecosystem functions, sediment cores were collected and compared within seagrass meadows of varying communities across sites of Unguja Island. The goal was to find out which seagrass traits are relevant indicators of carbon storage, and which environmental conditions constrain the storage of carbon in the sediments. Very fine sediments (<125 μm) were negatively correlated to organic carbon in the sediment. Leaf area index of seagrass was positively correlated to organic carbon content in the sediment, indicating an effect of particle trapping and retention. Root maximum length was the most important functional trait driving carbon storage, suggesting that rooting depth is of fundamental importance for carbon accumulation.

To conclude, TBFs can help to push seagrass research forward by the study of traits from the individual plant level, scaling up their effects on the seagrass community, interspecific competition and, lastly, ecosystem functioning. The individual trait responses of seagrass to environmental drivers, through adaptive processes, have fundamental consequences for interspecific competition and, ecosystem function. Changes in seagrass morphology can determine the outcome of interspecific competition for nutrients and light and, therefore, the final configuration of seagrass meadows. These traits of the species in the meadow ultimately determine the capacity of the meadow for carbon storage, which shows a prime example of how traits can affect important seagrass ecosystem functions.