For example, the water is deep and the coral reefs are far from the Burdekin River, which has the biggest floods but many dry years. By contrast, reefs grow at the mouth of some of the more predictable smaller rivers north of Port Douglas. When a flood plume reaches a coral reef, the freshwater alone can kill the corals, molluscs and other animals, irrespective of the silts or nutrients it carries. But at reefs or parts of reefs where there are abundant fish or sea-urchins, the latter can eat the seaweeds almost as quickly as they grow, leaving the area bare except for sparse and wispy shoots of new growth 17 . The approximate latitudes of the reefs and locations on the continental shelf are indicated on the edges of the boxes.
Seagrasses have roots and grow on sand and mud.
However, they are also vulnerable to the effects of runoff. Interpreting changes in seagrass areas poses similar difficulties to interpreting increases in seaweed abundance. This event led to starvation, deaths and emigration in an important dugong population that will take 25 years to recover 20 . While 'runoff' was clearly a major cause of this event, it remains to be established whether agricultural or other land use in the catchments emptying into Hervey Bay caused the plume to be more turbid, to cover a larger area, or to persist longer than it would have, prior to the area's development for agriculture. The seaweed and the seagrass stories recounted here thus do provide ammunition for the DPI catchment manager.