Spatial and Temporal Dynamics of Microphytobenthos Abundance from Remote Sensing: Heron Reef, Great Barrier Reef, Australia

Abstract

Microphytobenthos (MPB) comprise microbes inhabiting seafloor sediments that play important roles in aquatic ecosystem functioning. These microbial associations are highly variable in time and space and can mediate processes such as primary productivity, nutrient cycling, mineralization of particulate organic matter and trophic relationships. However, there is an absence of approaches to estimating coral reef MPB abundance distribution. Remote sensing can be used for large-scale mapping (e.g. >10s km) and for studying the dynamics of this variable in order to develop models and testable hypotheses regarding their ecological function. The aim of this research was to develop an approach for mapping and studying changes in MPB abundance in Heron Reef, Australia. This aim was addressed through three objectives: (1) to explore the relationship between Chlorophyll a concentration and spectral reflectance and to integrate this relationship with the Worldview-2 (WV-2) sensor, (2) to develop an approach to mapping the spatial distribution of MPB abundance over time using satellite image time-series with different spatial resolutions, and (3) to use the developed mapping approach to study the temporal and spatial dynamics of MPB abundance. In this thesis, Chlorophyll a concentration was assumed to be the proxy for MPB abundance; and MPB abundance was assumed to be the only controller of Chlorophyll a levels within benthic sediments of the reef. To address the first objective we measured in-situ underwater spectral reflectance from benthic sediments and same-point cores were extracted to estimate their Chlorophyll a concentrations. Regression models of Chlorophyll a concentration and spectral reflectance re-sampled to Worldview-2 (WV-2) sensor’s bandwidths were developed and the highest coefficient of determination was obtained in the blue spectral band of the sensor at 479 nm. This blue band regression model was suggested as a potential predictor of Chlorophyll a concentrations in Heron Reef benthic sediments. To address the second objective, we developed additional regression models between Chlorophyll a concentration and spectral reflectance (400-700 nm) with the later re-sampled to four satellite multispectral sensors’ blue bands: WV-2, Quickbird (QB), Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM). Models were applied to individual imaging pixels of corresponding satellite image time-series’ blue bands to derive MPB abundance maps. For validation we compared MPB abundance estimated from two TM and WV-2 image derived maps to field Chlorophyll a concentration data. Validation resulted in more accurate mean MPB abundance estimated from the TM image derived map than from the WV-2 one. The latter, however, was more advantageous for observing small MPB spatial patterns (~2-2.4m) in Heron Lagoon. Preliminary visual analysis of moderate spatial resolution MPB abundance time-series identified area-specific patterns in abundance that may be linked to herbivore grazing, tidal currents and reef geomorphology. The third objective was addressed by applying the regression models corresponding to TM/ETM sensors’ blue bands to corresponding blue band imagery so as to derive time-series of MPB abundance maps (i.e. MPB time-series) at inter-annual (1987-2011) and intra-annual (1991, 10 monthly images) intervals. The amount of Chlorophyll a per map was calculated resulting in total MPB abundance time-series for Heron Reef. Total MPB abundance time-series exhibited a slight increasing trend with a distinct seasonality, peaking in summer and hitting a trough in autumn/winter. Spatially trajectory analyses, which consisted of tracking Chlorophyll a concentration pixels over time in MPB time-series, derived two corresponding 1987-2011 and 1991 MPB trajectory maps in which each class represented averaged Chlorophyll a concentration trajectories over time. Trajectory analyses also suggested that the observed trend in the total MPB abundance time-series, was driven by a specific group of pixels that had a similar Chlorophyll a concentration trajectory and represented the largest area. Regression analyses between inter-annual MPB abundance time-series and tides, irradiance, photosynthetically active radiation (PAR), water temperature and winds showed no statistically significant relationship. This research resulted in the development of a rigorously validated method to map MPB abundance in Heron Reef at spatial and temporal scales never studied before in coral reefs. The method presented was useful in characterizing the spatial and temporal dynamics of MPB abundance using moderate spatial resolution TM/ETM imagery. Landsat imagery, in particular, can be used for the on-going monitoring of Heron Reef MPB abundance using the mapping method developed. The application of this method to other coral reefs with shallow and clear waters is feasible, provided new field-based regression models are developed for these environments because of the high spatial and temporal variability surrounding MPB abundance estimations.