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Indicators of Coastal Water Quality
Description of the Data
The flow of nutrients into coastal waters from land-based sources has seen a worldwide increase over the last decades. The resulting change in water quality has many potential impacts on coastal and marine ecosystems. Phosphorus and nitrogen contribute to enhanced algae growth, and subsequent decomposition reduces oxygen availability to benthic sea creatures like fish, shell fish, and crustaceans. Changes to nutrient loadings can also change the phytoplankton species composition and diversity. In extreme cases, eutrophication can lead to hypoxia—oxygen-depleted “dead zones”—and harmful algal blooms.
Measuring chlorophyll concentrations as an indicator of algae biomass may provide one tool to assess coastal water quality and its change over time. Here, we used chlorophyll-a concentrations derived from NASA's Sea-viewing Wide Field-of-view Sensor (SeaWiFS) to analyze trends over a ten year period (1998–2007). We attempted to identify near-coastal areas with improving, declining, and stable chlorophyll concentrations that can provide guidance for decision making in the context of environmental management.
This Web site provides a description of the global data set and how it was derived. Raster data are available for download in ESRI GRID and GeoTiff format, and the indicators of change are provided in tabular format. Ancillary data are provided in ESRI GRID and Shapefile formats.
| Table 1: Country/territorial offshore waters in which chlorophyll-a concentrations showed a statistically significant positive trend line for all annual average observations from 1998 to 2007, the corresponding increase in percent (average of 2005–2007 compared to 1998–2000 values), and average coastal concentrations (mg/cubic meter) in the first and last year of the time series. (Note: in some cases, such as Fiji and Sudan, the difference between start and end values is within the error bars of sensor accuracy and should be interpreted with caution.) | Table 2: Country/territorial offshore waters in which chlorophyll-a concentrations showed a statistically significant negative trend line for all annual average observations from 1998 to 2007, the corresponding decrease in percent (average of 2005–2007 compared to 1998-2000 values), and average coastal concentrations (mg/cubic meter) in the first and last year of the time serie. (Note: in some cases, such as French Polynesia and Hawaii, the difference between start and end values is within the error bars of sensor accuracy and should be interpreted with caution.) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Figure 1: Percentage of world-wide coastal grid cells with statistically significant trends in chlorophyll concentrations from 1998 to 2007 (including all annual observations) that showed positive versus negative change | Figure 2: Percent of countries where statistically significant trends in chlorophyll concentrations could be identified in at least some proportion of their coastal waters | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Figure 3: Trends in chlorophyll-a concentration in near-coastal waters of Vietnam and Cambodia. Colored pixels represent statistically significant changes over the 10 year time period from 1998 to 2007. Black pixels may have had annual fluctuations in chlorophyll-a concentrations, but they did not have statistically significant postiive or negative changes over the time period.
Methods
Yearly averages of chlorophyll-a concentrations (in ng/cubic meter) are based on annual composites* of SeaWiFS satellite data provided by the SeaWiFS Project, NASA/Goddard Space Flight Center and GeoEye. Data were downloaded from the Level-3 Browser in the form of HDF files at a resolution of 9 x 9 km. Level-3 products are derived from true-color images generated from sub-sampled, calibrated, Rayleigh-corrected level-2 data, which are derived from raw radiance counts by applying sensor calibration, atmospheric corrections, and bio-optical algorithms. For more details on data format specifications, see the SeaWiFS product information page.
HDF files were converted to ESRI GRID format using the Marine Geospatial Ecology Toolbox, version 0.4. Coastline buffers were clipped by country and Exclusive Economic Zone (EEZ). A layer of EEZ boundaries was obtained from the VLIZ Maritime Boundaries Geodatabase (Version 1) of the Flanders Marine Institute, Belgium.
For defining coastal zones, we excluded the first ten kilometers of coastal waters because of the potential for bottom-reflectance or suspended sediments affecting the satellite measurements in close proximity to the coast. We limited the extent of coastal zones to 100 km offshore as an arbitrary cut-off above which impacts from land based sources on oceanic eutrophication are unlikely.
We converted radiance values into chlorophyll-a concentrations using the scaling equation provided in the HDF header files. We multiplied concentrations by 1,000 to obtain ng/cubic meter that could be stored in integer grids for further processing without losing valuable levels of accuracy. We exported attribute information from the clipped global grids representing near-coastal zones, and calculated the percentage of change between 1998 and 2007 for each grid cell (i.e., time series). To account for year-to-year fluctuations in chlorophyll concentrations, we assessed the strength of the relationship between time and chlorophyll concentration with a linear regression for each grid cell. Percentages of change were then expressed and mapped for those areas with significant changes in concentration only. A detailed documentation with step-by-step procedures and additional global statistics can be found in the following document:
1.6 MB ) * Note: For regions with heavy seasonal cloud cover, there is a potential bias in the use of annual composites in that these composites make use of every cloud-free valid pixel in a year. Thus, less cloudy periods will be over-represented in the sample. If chlorophyll-a abundance is correlated with cloud cover in a given region - and it often is (e.g., coastal upwelling which brings phytoplankton to the surface in the Pacific Northwest also creates a cloud layer) - then the presence of clouds creates a sampling bias. This product does not address this issue, but in the future we plan to develop a product that does by using monthly composites instead of annual composites.
Data Download
top ^Raster data
The spatial data are provided in ESRI GRID and GeoTiff format. Raster cell sizes are 0.083333 degree decimal (about 9 kilometers at the equator). To download the zip files with accompanying metadata and documentation, click on the links below.
Annual composites in GeoTiff format (approximately 1.44 MB each, zip file)
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Tabular data
Based on the SeaWiFS annual grids, we created a large table that includes a time series from 1998-2007 of the chlorophyll-a concentrations for each grid cell. The grid cells are organized by country, and statistics are calculated to show the percent change from 1998-2007 and if the grid cell has a statistically significant trend in chlorophyll-a concentrations. The rows of the table also are linked to a sequence grid (see ancillary data) to facilitate the mapping of the trend values for selected countries and areas of interest. The documentation describing how to do this can be found in the "Indicators of Coastal Water Quality: Data, Methods, and Results" PDF above.
The tabular data are provided in Excel 2007 and CSV format. Because the complete dataset exceeds the row limit for earlier versions of Excel (about 439,000 rows), we have broken it into 8 files that can be downloaded separately. To download the data (with accompanying metadata and documentation), click on the link below.
- Change in Chlorophyll-a Concentration 1998-2007 (All Countries) (73.4 MB, Excel 2007)
- Change in Chlorophyll-a Concentration 1998-2007 (Alaska – Argentina) (11.4 MB, CSV format)
- Change in Chlorophyll-a Concentration 1998-2007 (Australia – Cameroon) (8.3 MB, CSV format)
- Change in Chlorophyll-a Concentration 1998-2007 (Canada - Cyprus) (15.4 MB, CSV format)
- Change in Chlorophyll-a Concentration 1998-2007 (Denmark - India) (11.3 MB, CSV format)
- Change in Chlorophyll-a Concentration 1998-2007 (Indonesia - Mayotte) (15.0 MB, CSV format)
- Change in Chlorophyll-a Concentration 1998-2007 (Mexico - Poland) (15.7 MB, CSV format)
- Change in Chlorophyll-a Concentration 1998-2007 (Portugal - Somalia) (14.8 MB, CSV format)
- Change in Chlorophyll-a Concentration 1998-2007 (South Africa - Yemen) (12.1 MB, CSV format)
Ancillary data
These data can be used in conjunction with the tabular data, as described in the documentation.
- Sequence grid (17.96 MB, ESRI GRID format)
- Centroids for sequence grid (249.17 MB, Shapefile format)
- Country buffers of near-coastal zones (3.83 MB, Shapefile format)
Citation and Acknowledgements
CIESIN/SEDAC would like to acknowledge the advice of Dr. Ajit Subramaniam of the Marine Biology Department of the Lamont-Doherty Earth Observatory at Columbia University in the development of these indicators. The data were developed by Steffen Foerster, Malanding Jaiteh, and Alex de Sherbinin.
Should you download and use these data, please ensure that any results are accompanied by the following data citation:
- Center for International Earth Science Information Network (CIESIN), Columbia University. 2009. Indicators of Coastal Water Quality: [Title of Constituent Data Set]. Palisades, NY: Socioeconomic Data and Applications Center (SEDAC), Columbia University. Available at http://sedac.ciesin.columbia.edu/es/seawifs.html. [Date downloaded]
