Marine urbanization is a term that describes the increasing proliferation of structures such as piers, jetties, marinas, sea walls and other coastal defences in the marine environment. Martino Malerba, Craig White and Dustin Marshall from the Centre for Geometric Biology are proposing that this type of urban spread into the ocean could rob local systems of some of their productivity, changing local food web structure as well as function.

Many artificial structures, including floating platforms, barges, piers, pontoons seawalls and port quays, decrease the access of direct sunlight into the water. This means that instead of ecological communities dominated by ‘energy-producing’ seaweeds that require light to photosynthesise, there tends to be a shift toward dense assemblages of ‘energy-consuming’ filter-feeding invertebrates. The research team are particularly interested in finding out how this additional filter-feeding biomass affects energy flow and the productivity of coastal food webs.

Martino and his colleagues have combined data from field surveys, laboratory studies and satellite data to analyse total energy usage of invertebrate communities on artificial structures in Port Phillip and Moreton Bays. The team then used estimates from other studies to estimate the amount of primary production required to support the metabolic demands of the entire filter-feeding biomass living on artificial structures of all main commercial ports worldwide: the ‘trophic footprint’.

In order to do this, Martino and colleagues first used satellite photos to estimate how much area available for colonisation is created by artificial structures in Port Phillip and Moreton Bays. They then used field surveys to estimate the total invertebrate biomass occurring on all artificial structures in the two bays. In Port Phillip Bay, the estimated biomass on artificial structures is the equivalent of 3,151 female African bush elephants.

The next step was to transport communities back to the laboratory to measure mean daily energy consumption per unit area and then scale this up to artificial structures within the whole bay.  Based on their estimates they found that biomass on artificial structures can consume between 0.005% and 0.05% of the total yearly energy production in Port Phillip Bay and Morton Bay respectively.  This means that each square metre of artificial structure will consume 6 to 20 m² of marine primary productivity in these bays.

When they went through essentially the same process using data available from the scientific literature, the team found, on average, each metre of port consumes 26 m^²of ocean primary productivity.  But there are stark differences between ports in different parts of the world. For example, one square metre of artificial structure in cold, highly productive regions can require as little as 0.9 m² of ocean (e.g. St. Petersburg, Russia), whereas a square metre of artificial structure in the nutrient-poor tropical waters of Hawaii can deplete all of the productivity in the surrounding ~120 m^².

Martino, Craig and Dustin point out that a large percentage of ocean shoreline is now modified by engineering, with associated impacts on primary production. Burgeoning coastal human populations are expected to increase demands on fisheries in the future and food web productivity is already forecasted to decrease, due to the effects of climate change. Understanding the impacts of marine urbanisation on food webs is becoming increasingly important so that design of artificial structures minimises impacts on food webs and remediation efforts can be undertaken if necessary.

This research is published in Frontiers in Ecology and the Environment.