Mapping coral reef primary production and calcification from a light-use efficiency model with in-situ and remotely sensed data

Abstract

Monitoring coral reef primary and carbonate production is critical to understand changes in ecosystem function over time and between different reef systems. This is essential with increasing anthropogenic stresses from coastal development, overfishing, pollution, and ocean warming and acidification. However, monitoring these processes at spatial scales relevant to on ground management showing details within individual reefs (100’s m2) and across reef systems (1000’s km2) has been difficult due to lack of biogeochemical models that incorporate physical and biological factors for scaling with remote sensing. There is potential to adopt existing models that use absorbed photosynthetically active radiation (APAR) for estimating productivity, such as the light-use efficiency (LUE) model, which has been successfully applied in terrestrial and open ocean systems.The aim of this thesis was to develop methods for applying the LUE model to coral reef environments with field and remotely sensed data collected at multiple spatial scales. The objectives were to: (1) assess variability in community net productivity and calcification rates for algae dominated reef flats and a seagrass community, (2) assess variability of photosynthetic and calcification efficiencies based on APAR for multiple functional-groups: coral, non-calcareous algae, calcareous algae, sediment, and seagrass, and (3) apply the LUE model to map gross productivity and net calcification using multi-spectral imagery. This thesis focused on two reef systems: Heron Reef, southern Great Barrier Reef and Saipan Lagoon, Commonwealth of the Northern Mariana Islands, which have multi-spectral imagery available for model application.The first chapter of this thesis provided a brief overview of productivity and calcification processes in coral reef systems, and current limitations in monitoring these processes with remote sensing. In Chapter 2 (Obj. 1), field measurements of community net productivity and calcification are reported for algae dominated reef flats and a seagrass community. This study provided the first baseline metabolism and calcification rates for Saipan Lagoon. Both the reef flats and seagrass community had high net productivity that was associated with the dominant presence of algae at all sites.In Chapter 3 (Obj. 2), the theoretical basis for the light-use efficiency model is introduced, and how the model used the relationship between irradiance and photosynthesis to estimate productivity. The relationship between productivity, calcification, absorptance, and irradiance are assessed for the reef flats and seagrass community surveyed in Ch 2. Photosynthetic efficiencies for both reef flats were comparable to previous measurements of light-use efficiency for coral and macroalgae. The seagrass community had the lowest photosynthetic efficiency, due to the high absorptance at the site. Results from this study highlighted the need for field-derived estimates of LUE at scales necessary for remote sensing applications. However, further research is needed to understand variability in LUE parameters from organisms to community scales, and from day to seasonal scales.In Chapter 4 (Obj. 3), maps of gross productivity and net calcification were generated for Heron Reef with WorldView-2 imagery, and for Saipan Lagoon, a Planet Dove mosaic. The maps were based on the LUE model parameterized with the field values measured in Ch. 2 and 3. The model application was tested with a constant LUE, representative of the reef or seagrass community applied across the entire reef system, and an individual LUE for each functional-group mapped. Applying a constant LUE, at the community scale, provided a logical pattern of high productivity and calcification in coral/algae areas. However, due to the limited field data available for parameterizing LUE for sand, productivity and calcification were overestimated for the mapped area. Limitations to using multi-spectral imagery, and water column corrections to obtain bottom absorptance were also identified. Planet Dove imagery did not have sufficient spectral resolution or signal-to-noise ratio (SNR) for applying the LUE model, and for mapping benthic functional groups. LUE model application and mapping benthic functional groups required higher spectral resolution, such as the WorldView-2 sensor, which has additional wavebands in the shorter wavelength range. Hyperspectral sensors may be more suitable for LUE model applications. In chapter 5, results are summarized for chapters 2-4, and several recommendations were identified to improve application of the LUE model. This included characterization of water optical properties to appropriately correct remotely sensed imagery to bottom reflectance, and use of remotely sensed imagery with high SNR and increased spectral resolution. Additional parameters characterizing light attenuation within the seagrass canopy or reef structure, and environmental parameters (i.e temperature, water flow) that influence productivity are also needed to improve the model. In addition, collection of in-situ productivity and calcification rates with absorptance that target multiple community or functional-groups at high temporal resolution, e.g. hourly rates over a 24-hr period or longer, are needed to (1) effectively parameterize the LUE model, (2) evaluate spatial and temporal variability of LUE, (3) develop a monitoring network that standardizes field data collection across multiple reef systems, and (4) use in validation of remotely derived productivity and calcification maps.