The seasonal deposition of seaweed in Ralphs Bay near Lauderdale Primary School is the result of a combination of factors that present themselves often in Spring, this generally includes an increase in nutrients in the water column and an increase in sunlight as well as the right kind of southerly swells to push seaweeds in toward Lauderdale and above the high tide line. The resulting anaerobic decomposition of organic materials (mostly filamentous brown algae and Ulva ie. sea lettuce) causes a strong odour predominately attributed to the volatile sulphur compounds produced, particularly Hydrogen Sulphide.

How does it happen?

Lauderdale faces a vast sheltered basin of shallow water that is subject to occasional swells. In spring, there are lots of nutrients in the water column that have yet to be soaked up, and at the same time you get a big increase in sunlight that promotes growth, including growth of seaweeds on the seabed as well as phytoplankton in the water column. Much of the Ralphs Bay, and beyond further into the estuary, is shallow enough for light to reach the seabed and promote algal growth. At depths of 20m or less this can be prolific in spring. This process can involve a variety of species, many of them being red algae that flourish at lower light levels. They grow on anything from reef to the small shell particles out on the sandy seabed typical of that area. When/if you get the right kind of late spring swells, with a lot of seabed surge, all the weeds that aren’t strongly anchored to something like reef end up being moved and often end up piled on beaches. At times up to a few metres deep and many metres wide in some locations. It is typically a natural process, albeit somewhat enhanced by nutrient inputs from human sources and the hydrodynamics of the Derwent Estuary as a lot of water from the catchment spins around into the bays. At this stage it is nothing out of the usual, although worth monitoring in the longer-term to see if such events increase as nutrient inputs to the region increase with increasing urbanisation and development of salmonid farms offshore etc.

The odour

The smell is made up of multiple contributors. The initial smell after deposition can be attributed to the high amounts of iodine contained in seaweed. However, the most unpleasant of the odours are produced by the emission of volatile sulphuric compounds, such as hydrogen sulphide and methyl mercaptan, as the material breaks down under anaerobic conditions. Another contributor to the smell can be the invertebrates associated with the breakdown of the material that smell even more, however the whole process of decay is doing good things for the coastal environment.

Environmental Importance

Importantly the deposition of seaweed is a natural seasonal process and as such there are some species in the system that actually depend/ thrive on the presence of such nutrient deposits. Importantly, whilst temporarily unpleasant for the community, there are a number of invertebrates and other animals that rely on this process to survive.

Fun fact: Seaweed is a type of algae with two key characteristics: they are macroscopic and marine.