Invasive alien species in urbanised marine environments: management applications of natural and manipulated salinity stress

The development of the world’s oceans is not only changing the physical appearance of our coastlines, but also the marine communities that inhabit them. Increasing human activity in the seas has contributed to the spread of a vast number of marine invasive non-native species (NNS), which can have serious environmental and economic impacts in their introduced regions. In order to reduce the spread of these species, research into ways to cost-effectively monitor the arrival of new species and control or eradicate harmful species is needed. NNS are especially prevalent as fouling species on artificial structures within urbanised environments, such as harbours and marinas, where the high level of disturbance and changes in environmental conditions may facilitate their establishment. This thesis aims to investigate the effects of natural and manipulated changes in salinity on NNS within these environments, to address knowledge gaps in the management of NNS in Scotland.

In order to assess the long-term patterns of individual marine NNS establishment and spread, this thesis begins by updating data on the baseline distribution of marine and brackish NNS in Scotland. In 2016, rapid assessment surveys (RAS) were carried out in eighteen of the largest marinas in mainland Scotland, finding eighteen fouling NNS, triple the number of species detected in 2008 when marinas were last surveyed. The average number of species per site has also increased, rising by 3.2 species over the ten year period from 2006 to 2016. Since the last compilation of marine NNS records in 2012, increased survey efforts across the country have seen an additional 1006 presence records added to a synthesised list of marine and brackish NNS. These records show that there have been a number of new species introduced to Scotland in this period, with the number of recorded marine NNS now totalling 32 species, 22 of which are considered established or likely to be established. Sites identified in this study as having high abundances of NNS should be incorporated into strategic monitoring to allow for early detection of NNS new to Scotland.

The influence of salinity variation and other abiotic factors on variation in marina fouling communities was then evaluated. Patterns of NNS diversity were investigated among marinas found on a spectrum of high to low salinity. Field surveys were conducted to assess how native and non-species diversity contributed to fouling community composition on floating pontoons, with primary and secondary data collected on temperature and salinity, site location, marina features and river parameters. Fouling communities are dominated by native species, although NNS occupied more than 50% cover in one site, and over 10% cover in another three sites. NNS and native species diversity were influenced by salinity conditions within a site, with sites further away from rivers having significantly more diverse fouling assemblages and higher numbers of NNS. This data could be used to develop a profile of sites with higher susceptibility to NNS colonisation, allowing for more cost-efficient assignment of monitoring effort.

Finally, two experiments were conducted to assess the effectiveness of treatment with hypo-saline waters as a tool for managing biofouling and controlling fouling NNS. Immersion in water with a salinity lower than 10 was shown to reduce biofouling load, resulting in loss of 20 - 50% of biofouling cover. Variation in species’ tolerance to osmotic change resulted in a shift in community composition in the eight weeks following treatment. Susceptibility to low salinity treatment also varied between NNS, such that growth of some NNS actually increased following treatment with hypo-saline water, potentially due to reduced competition from dominant native species. Effectiveness of treatment with freshwater was lessened in sites where communities were exposed to low salinity periods, with no effect of treatment on communities in the site with high freshwater input. These results support previous research indicating the potential of osmotic shock as a method for reducing biofouling, but highlight the potential for unintended consequences if community and NNS tolerance are not given due consideration.

These findings highlight opportunities to use the natural salinity variation in Scottish urbanised waters to guide development of NNS monitoring and management strategies. Increased attention is being given to marine NNS management in Scotland, as opportunities for biosecurity planning emerge in the development of regional marine plans, and as marine industries are increasingly becoming aware of their responsibilities to follow best practice guidance in NNS management. The results of this study should be used to inform biosecurity plan development, and to help develop a strategic framework for national monitoring of NNS.