Caroline Petus

A novel approach to model exposure of coastal-marine ecosystems to riverine flood plumes based on remote sensing techniques

Increased loads of land-based pollutants are a major threat to coastal-marine ecosystems. Identifying the affected marine areas and the scale of influence on ecosystems is critical to assess the impacts of degraded water quality and to inform planning for catchment management and marine conservation. Studies using remotely-sensed data have contributed to our understanding of the occurrence and influence of river plumes, and to our ability to assess exposure of marine ecosystems to land-based pollutants. However, refinement of plume modeling techniques is required to improve risk assessments. We developed a novel, complementary, approach to model exposure of coastal-marine ecosystems to land-based pollutants. We used supervised classification of MODIS-Aqua true-color satellite imagery to map the extent of plumes and to qualitatively assess the dispersal of pollutants in plumes. We used the Great Barrier Reef (GBR), the worlds largest coral reef system, to test our approach. We combined frequency of plume occurrence with spatially distributed loads (based on a cost-distance function) to create maps of exposure to suspended sediment and dissolved inorganic nitrogen. We then compared annual exposure maps (2007-2011) to assess inter-annual variability in the exposure of coral reefs and seagrass beds to these pollutants. We found this method useful to map plumes and qualitatively assess exposure to land-based pollutants. We observed inter-annual variation in exposure of ecosystems to pollutants in the GBR, stressing the need to incorporate a temporal component into plume exposure/risk models. Our study contributes to our understanding of plume spatial-temporal dynamics of the GBR and offers a method that can also be applied to monitor exposure of coastal-marine ecosystems to plumes and explore their ecological influences.

Effects of reduced water quality on coral reefs in and out of no‐take marine reserves

Near-shore marine environments are increasingly subjected to reduced water quality, and their ability to withstand it is critical to their persistence. The potential role marine reserves may play in mitigating the effects of reduced water quality has received little attention. We investigated the spatial and temporal variability in live coral and macro-algal cover and water quality during moderate and major flooding events of the Fitzroy River within the Keppel Bay region of the Great Barrier Reef Marine Park from 2007 to 2013. We used 7 years of remote sensing data on water quality and data from long-term monitoring of coral reefs to quantify exposure of coral reefs to flood plumes. We used a distance linear model to partition the contribution of abiotic and biotic factors, including zoning, as drivers of the observed changes in coral and macro-algae cover. Moderate flood plumes from 2007 to 2009 did not affect coral cover on reefs in the Keppel Islands, suggesting the reef has intrinsic resistance against short-term exposure to reduced water quality. However, from 2009 to 2013, live coral cover declined by ∼ 50% following several weeks of exposure to turbid, low salinity water from major flood plume events in 2011 and subsequent moderate events in 2012 and 2013. Although the flooding events in 2012 and 2013 were smaller than the flooding events between 2007 to 2009, the ability of the reefs to withstand these moderate floods was lost, as evidenced by a ∼ 20% decline in coral cover between 2011 to 2013. Although zoning (no-take reserve or fished) was identified a significant driver of coral cover, we recorded consistently lower coral cover on reserve reefs than on fished reefs throughout the study period and significantly lower cover in 2011. Our findings suggest that even reefs with an inherent resistance to reduced water quality are not able to withstand repeated disturbance events. The limitations of reserves in mitigating the effects of reduced water quality on near-shore coral reefs underscores the importance of integrated management approaches that combine effective land-based management with networks of no-take reserves.

Using MODIS data for mapping of water types within river plumes in the Great Barrier Reef, Australia: Towards the production of river plume risk maps for reef and seagrass ecosystems

River plumes are the major transport mechanism for nutrients, sediments and other land-based pollutants into the Great Barrier Reef (GBR, Australia) and are a major threat to coastal and marine ecosystems such as coral reefs and seagrass beds. Understanding the spatial extent, frequency of occurrence, loads and ecological impacts of land-based pollutants discharged through river plumes is essential to drive catchment management actions. In this study, a framework to produce river plume risk maps for seagrass and coral ecosystems, using supervised classification of MODIS Level 2 (L2) satellite products, is presented. Based on relevant L2 thresholds, river plumes are classified into Primary, Secondary, and Tertiary water types, which represent distinct water quality (WQ) parameters concentrations and combinations. Annual water type maps are produced over three wet seasons (2010-2013) as a case of study. These maps provide a synoptic basis to assess the likelihood and magnitude of the risk of reduced coastal WQ associated with the river discharge (river plume risk) and in combination with sound knowledge of the regional ecosystems can serve as the basis to assess potential ecological impacts for coastal and marine GBR ecosystems. The methods described herein provide relevant and easily reproducible large-scale information for river plume risk assessment and management.

Water quality monitoring in Basque coastal areas using local chlorophyll-a algorithm and MERIS images

Accurate estimation of chlorophyll- a (chl- a ), a proxy of the eutrophication risk, is necessary in coastal areas for the assessment of water quality in accordance with European Directives. Local parameterization of remote sensing algorithms is useful to cope with the variability and specificity of optically-active in-water constituents. Using the Bay of Biscay coastal waters, affected by Basque river runoffs, as a case study, the objectives of this investigation are to: 1. develop an empirical algorithm to estimate water surface chl- a for the optically-complex Basque coastal waters; 2. explore the influence of suspended matter, phytoplankton species, and pigment content on the algorithm developed for medium resolution imaging spectrometer instrument (MERIS) imagery; 3. compare the local algorithm to three ocean color algorithms (OC4v6, Gitelson’s algorithm, and the OC5); and 4. apply the local algorithm to the MERIS images. For this purpose, two surveys were undertaken within the study area, the Batel-1 survey in 2007, and the Batel-2, in 2009. The empirical algorithm was developed with remote sensing reflectances ( R rs ), undertaken with a TriOS field spectrometer, and chl- a measured in situ from the Batel-2 survey. The algorithm was not affected by different concentrations of suspended matter in surface waters, within the range from 0.0 to 6.6     g · m − 3 . There was no significant effect of 23 accessory pigments found in the area on the algorithm. Eighty-four R rs and chl- a measurements from the Batel-1 survey were used to validate the local algorithm and to compare it with output of the other algorithms. The local algorithm provided the lowest root-mean-square difference ( RMS = 1.7     mg · m − 3 ), the best correlation with the observed data ( R = 0.8 ), together with the best slope-intercept combination between predicted and observed chl- a ( slope = 0.5 , intercept = 0.6 ). The chl- a algorithm developed here for MERIS imagery can assist in the assessment of water ecological status in the southeastern part of the Bay of Biscay, in a cost-effective manner.