4. Invasive Species in the Ocean
Can we see marine invasive species from space?
The answer is: some of them, depending on their nature and size.
Images derived from remote sensing systems typically average the information they detect over tens or even hundreds of square meters. Some commercial satellites offer a spatial resolutions of little less than 3 m, such as SPOT (2.5 m), IKONOS (1 m), or Quickbird (0.61 m). Aerial photography can theretically provide images with a spatial resolution of as little as a few centimeters.
Have a look at the two images of the North Sea below. One is a satellite image with a medium resolution of 30 m, the other one is an aerial photograph with a spatial resolution of < 1 m. Which species do you think could be distinguished from any of these? (Please note that the resolution of the images has been reduced to minimise the time needed to load the image).
Landsat 7 ETM image of the German North Sea Coast and the Wadden Sea.
Source: Eurimage 2002, provided by the Wadden Sea National Park of Lower Saxony
Aerial photograph of the Dornumer Nacken, German North Sea, aquired on 12.09.2008.
Source: Wadden Sea National Park of Lower Saxony
The advantages of the two platforms (satellite and aircraft) are explained in some more detail in the chapter about Detection and monitoring of marine pollution using remote sensing techniques.
The detection of marine invasive species in optical images is generally rather complicated and needs a lot of ground truthing, i.e. field work to validate that what we have seen on the image is really what we think it is. Single species could possibly be detected on aerial photographs with a very high spatial resolution. This would however largely reduce the size of the area under observation.
While it is nearly impossible or at least not cost effective to monitor marine invasive species by means of remote sensing, there are examples of applications of remote sensing that can be indirectly used.
(1) The pathways of invasive species in the oceans are often dominated by ocean currents that in turn can be followed by satellite imagery.
(2) Although it is not possible to distinguish different marine plant species from satellite imagery, many plant species can be observed by means of detecting the colour of the ocean.
