Explanation Surrounding Succession and 'Level of Biofouling'
The Commonwealth of Australia devised a simplistic, but pragmatic method of assessing the risk and likelihood of vessels or infrastructure containing Invasive Marine Species (IMS) of concern.
The colonisation and accumulation of biofouling on surfaces submerged in sea water follows a very complex process from the time of settlement of the initial microscopic organisms through to the establishment of macroscopic biofouling organisms.
Figure; The simplistic, but pragmatic approach to assessing biofouling risk using three levels of biofouling or succession.
Primary biofouling begins the moment a vessel/structure’s hull is submerged in sea water with the immediate biochemical and bacterial conditioning followed by bacterial, diatom, protozoan and multi-cellular colonisation.
In its simplest form, the assemblage of biofouling accumulated over a period of time (e.g., such as the in-service period of a vessel/structure) can arguably be classified into three main levels. These categories (primary, secondary, and tertiary levels of biofouling; see Figure above) can be used to broadly classify a biofouling community at the time of observation. However, it is important to emphasise that these three categories are not entirely definitive as they constantly evolving and tend to overlap, particularly secondary and tertiary levels of biofouling.
Initially conditioning and colonisation of microscopic organisms provide an ideal substrate for more visible (macroscopic) organisms such as fine filamentous algae, some of which are resistant to the toxic biocides contained in anti-fouling coatings.
The establishment of these organisms tends to provide a suitable, but not necessarily mandatory substrate for the settlement of secondary biofouling organisms, which tend to be the most dominant and frequently encountered biofouling organisms on vessel/structure hulls. Secondary biofouling communities are more likely to progress towards tertiary biofouling, particularly in niche areas of vessels/structures that are protected from strong hydrodynamic forces or when vessels remain stationary for long periods of time, or simply the longer the in-service period. More importantly, the greater the level, age and complexity of biofouling, generally the greater the likelihood that IMS of concern will be present and mature.