Notes from Swimwell video chats:  Where Peter and Charlie try to answer some of the most common questions asked by members of our groups.

Charlie Evans, August 2020

As a horticulturist I have some understanding of the Nitrogen cycle and as an open water swimmer and coach I also have a keen interest in water quality in our Bay.

Port Phillip Bay is a biologically dominated system driven by phytoplankton and it is easy for the fine balance to be lost.

Excess nutrient – causes an increase in the number of phytoplankton which limits oxygen to other species and causes a build-up of ammonium which is trapped in the water

Low Nutrient – causes a reduction in the number phytoplankton and sediment remains toxic and releases nitrogen

Other Important general measures of water quality – Nitrogen, phosphorous and Silicate levels.

  • When de-nitrification efficiency in Port Phillip Bay is in correct balance Phytoplankton remain in balance and the sediment remains neutralised.
  • An increase of nutrients into the water column caused by exotic flora and fauna, inefficient farming practices (fertilizers do not help), storm water outfall and heavy industry, causes an increase in nutrients and therefore phytoplankton numbers increase.
  • Marine pests can also exacerbate the problem often creating imbalance and causing de-oxygenation to occur. For example, Pacific Stars and some jelly species will have this effect.
  • Sea grass meadows also fix nitrogen and carbon to their bio system which is then processed by the plant and released in a neutral state.
  • Climate change is causing warmer than usual water temps across the globe which can exaggerate the imbalance

Water clarity – Is the result of total suspended solids and turbidity. Salt, metals, algae, sediment contamination, water temperature and faecal contamination all influence clarity.

Rainfall – After heavy or persistent rainfall Enterococci bacteria, and including viruses and protozoa, e coli, human and pet waste, and water overflows make their way to the bay. After seasons of high rainfall, 2020 for example, the e coli often becomes reduced as the system has been effectively flushed. Often after long dry periods, the first major rainfall can cause the most challenging water quality

  • The EPA does not measure during the cooler months and bases their public information on data collected over time and historical averages including after rainfall.
  • Research drain, creek, river and general outfall locations and type of industry in the area before choosing to swim after heavy rainfall
  • Choose an area that is flushed regularly to the open water by strong tidal flow, being closer to the open ocean is usually best
  • Often the only sensible advice is to use your best judgement and choose your location based on an educated guess and your own anecdotal evidence 
  • Westernport Bay has a different system ruled by tidal flow and much of the information above does not relate at that location

Leave a Reply