|Seagrasses are ecosystem engineers that provide important ecological functions and societal economic values. Examples of the services that seagrasses provide are: sediment and coastal stability; maintenance of water quality; primary productivity for coastal ecosystems; fisheries nursery habitat; food for large herbivores; food-webs for complex marine communities; fisheries habitats; and carbon sink. They help minimise the costs of foreshore protection and maintain and support both tourism and fisheries economies. Different factors are implicated in causing the decline of seagrass ecosystems, but human activities are clearly identified as one of the major causes of seagrass decline in the world.
Humans affect this ecosystem via physical damage (e.g., harbour developments, trawling, aquaculture), introduced species, global change, and pollution (e.g., sediments, nutrients, wastewaters, herbicides, heavy metals, petrochemicals). In New Zealand, sediment is the most pervasive seagrass stressor and the most prominent cause of seagrass decline. The goal of this PhD was to determine sediment effects on the seagrass Zostera muelleri in terms of light attenuation and substrate physico-chemical alteration. Within this research framework, provision of assistance for successful seagrass restoration was also considered.
The principal research question for this project was to evaluate how sediment affects seagrasses and the project hypothesis was that sedimentation affects seagrass by altering the light climate, physically smothering the plants and modifying substrate physico-chemical composition.
An extensive global literature review was undertaken to improve understanding of the international body of knowledge on the effects of sediment upon seagrass. Field surveys, field experiments and mesocosm experiments were used to evaluate the research objective. Field experiments were undertaken in Pāuatahanui Inlet, New Zealand. This inlet provides a wide range of seagrass cover, historical seagrass sites and substrate conditions, which makes it an excellent field laboratory to test hypotheses. Mesocosm experiments were undertaken at the University of Waikato Marine Field Station in Tauranga.
A series of observations and experiments investigated the relationship between Z. muelleri growth, light climate and substrate properties. Initially, correlations between receiving irradiance, substrate physicochemical variables and Z. muelleri traits in Pāuatahanui Inlet, were explored using an observational seasonal survey. A series of experiments followed, that used field and mesocosm-based methods that allowed deeper analysis of how sediment affects light climate and substrate properties at seagrass habitats. These results provided new insights into conditions under which seagrass declines or is unable to re-establish. The observational-based field study was undertaken in three habitat types: historical seagrass habitat, existing seagrass habitat and potential seagrass habitat and involved two field campaigns in winter and summer. A variety of substrate physicochemical variables including substrate grain size, bulk density, redox profiles, porewater nutrients, dissolved metals, receiving irradiance and temperature were measured as well as Z. muelleri traits such as percent plant cover, rhizome length, shoot density leaf width and length. Significant differences of substrate properties were observed between deteriorated historical habitat substrate and existing seagrass habitats and potential seagrass habitats. Increased substrate muddiness and consequent unfavorable rhizosphere conditions were implicated as causes of seagrass decline or failure to recolonize historical habitat. The results suggested for the multi-stressor effects of sediment on seagrasses, with both substrate suitability and submerged light climate for seagrass being detrimentally affected.
However, despite considering a wide range of substrate properties and irradiance, the exact mechanisms of seagrass decline could not be extracted from the data collected in the observational field survey. Further manipulative mesocosm experimentation was expected to allow more conclusive inferences to be drawn on the influence of substrate physicochemical factors and irradiance on seagrass growth and persistence. A factorial mesocosm experiment was conducted to elucidate the links between these. Two irradiance treatments; low (6.3 mol m⁻² d⁻¹) and very low (2.3 mol m⁻² d⁻¹), were crossed with two substrate treatments; historical substrate (42 % mud) and existing substrate (20 % mud). Seagrass growth was monitored for six weeks. Belowground biomass and rhizome growth were significantly reduced by substrate muddiness but were unaffected by irradiance. However, shoot growth was significantly affected by reduced irradiance and increased substrate muddiness as well as the synergistic interaction between both these parameters. Results suggest that Z. muelleri inhabiting muddy substrates has an increased irradiance demand to deal with adverse rhizosphere conditions and specifically to oxygenate the rhizosphere. Therefore, interactions between substrate and light climate, which are both affected by fine sediment pollution, should be considered when determining light thresholds for seagrass survival.
In order to further investigate the effects of site and irradiance on seagrass, a field transplanting experiment was undertaken across the previously characterised habitats in the Pāuatahanui Inlet. The aim of this experiment was to disentangle substrate effects from other effects such as light climate and smothering. As the experiment progressed, some challenges to its successful completion emerged. Firstly, it proved impossible to reliably relocate some of the transplanted sprigs, which impeded the planned comparisons. Secondly, an incursion of the filamentous green algae Chaetomorpha ligustica smothered approximately half of the quadrats of one of the treatments. This is the first time, negative impacts of this species upon meadows of the New Zealand seagrass Zostera muelleri has been reported. Chaetomorpha ligustica can easily be misidentified in the field and genetic tests are required to identify this species. Hence, the need for careful identification of this green macroalga blooms in future as well as further research on growth requirements and origins of strains is desirable as it may play an important role on seagrass loss. Outcomes from this transplanting experiment allowed the conclusion to be drawn that the cumulative effect of rhizosphere deterioration, lower irradiance and close location to a source of natural sediment input during events such as storms may be the cause of the inability of seagrass to re-establish at Pāuatahanui Inlet in historical seagrass habitat.
The last experiment of the project aimed to compare the ability of the seagrass to carry out photosynthesis both in air and in water as this is potentially important for determining its vulnerability to enhanced water turbidity. To compare photosynthetic rates, oxygen (O₂) flux in water, CO₂ flux in air, and pulse amplitude modulated (PAM) fluorometry in both air and water were utilized. In water, “gross” photosynthetic O₂ evolution (GPS) as oxygen exchange averaged 2.24 μmol O₂ m⁻²s⁻¹, leaf respiration rates averaged 0.44 μmol O₂ m⁻²s⁻¹ and saturation irradiance 115 μmol photons m⁻²s⁻¹. In air, CO₂ showed light saturated gross photosynthesis of 2.26 μmol CO₂ m⁻²s⁻¹, respiration rates of 0.7 μmol CO₂ m⁻²s⁻¹ and saturating irradiance 286 μmol photon m⁻²s⁻¹. Compensation irradiance (Ec) is 22 μmol photons m⁻²s⁻¹ and 140 μmol photons m⁻²s⁻¹ when submerge and emerge showing higher photorespiration when emerged. Potential production of intertidal seagrass under submerged and emerged conditions was modeled across tidal cycles using experimental gas exchange results and field measured irradiance, using two scenarios; a high tide scenario 1 when high tide coincided with midday and low tide scenario 2 when low tide did. Respiration rate differed little between scenarios, and approximately similar amounts of net photosynthesis were predicted for emerged and submerged periods. In contrast emerged net photosynthesis was 25 times greater than submerged in the low tide scenario. These results support previous studies that have reported emerged photosynthesis as a mechanism to mitigate degraded submerged light climate, and to contribute to seagrass production estimates.
Lastly, a synthesis of new knowledge gained through this thesis, together with recently published literature is presented, which develops a new paradigm for understanding the interactive and cumulative effects of sediment on seagrass. Of particular importance are the complex interactions between irradiance and substrate muddification. This research suggests that a nuanced interpretation of fine sediment effects on seagrass, growth and persistence needs to be developed that is sensitive to the specific estuary exposure to the pollutant. Future directions for research are also suggested, which aim to build upon the research presented in this thesis and further advance understanding of the physicochemical drivers of seagrass Zostera muelleri loss. The information gathered from the research is available to help new methods of seagrass restoration development. This research provided evidence that enriches our knowledge of seagrass, especially estuarine seagrass ecosystems in New Zealand and this will provide an opportunity to create tools for better management of water quality and quantity targets within New Zealand to help maintain and hopefully restore this important ecosystem.