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.)|
|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|
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.