Green seas in the city

Seasonal green water on the Seattle waterfront

If you’re a barnacle, clam, mussel, or any other filter feeder in a temperate coastal city, you’re likely to be highly attuned to the changing of the seasons and the bounty of energy that arrives seasonally with the blooming of phytoplankton in spring.

Just as terrestrial plants at high latitudes explode each spring with new leafy growth and flowers, so too do photosynthetic plankton. Their proliferation in the spring sunshine turns urban waters a deep, soupy green. This gives filter feeders a reason to rejoice, as they’re doused in a profusion of food. Check it out:

Why is everyone so wild for phytoplankton? Orange sea cucumbers shove plankton-covered tentacles into their mouths. Serpulid worms collect passing food particles with their colorful branchial crown. Tubesnout fish hover in a milieu of green plankton, sucking in food as they go. Barnacles frantically fan the water with their feathery feeding appendages in an effort to get the most from the surrounding bounty while it lasts.

Sea cucumber

Phytoplankton account for approximately 90% of primary productivity in the oceans globally. In coastal areas, marine macroalgae (seaweeds) also contribute significantly to primary production. Together, these photosynthesizers are responsible for fueling the food web, supporting the multitude of species that live in shallow coastal seas.

It’s a seasonal drama that repeats annually with an invariably boom-bust pattern to which marine life are highly adapted. Boom periods in spring are the time to take up as much energy as possible and store it in body tissues or send it off with offspring.

Within weeks, the b(l)oom will have run its course. The water will once again become clear and blue, making each fanning motion a bit less lucrative for barnacles and their filter-feeding compatriots. As dying plankton sink to the bottom and blanket the seafloor, detritivores like sea cucumbers may continue to lie about in satiation. But for them too, the bounty will only last so long. It’ll be back to life as usual in the urban seascape before long.

Urban anemones in spring

Marine biofuels

Given the immense energy potential of phytoplankton and their macroscopic relatives – the seaweeds, marine primary producers have recently caught the attention of the energy industry and gained recognition as a potential future source of biofuel. Research initiatives on marine biofuels are popping up rapidly around the world, with considerable interest from the shipping and aviation industries, governments, and sustainability-oriented foundations.

One area that has received less attention thus far however, is the considerable potential of urban seaweeds to serve as rich cultivation grounds for biofuel production.

Urban marine environments are notoriously rich in nutrients such as nitrogen and phosphorous, which are key growing requirements for algae.

Since seaweed farming does have environmental consequences when carried out on a large scale, relatively pristine marine environments along rural coastlines or in the open ocean truly are NOT ideal locations to scale up biofuel production.

Urban environments, on the other hand, offer a highly sustainable alternative, as they are already heavily impacted, nutrient rich, and human-dominated.

It’s early days, but experimental small-scale seaweed farming is underway in Singapore and selected other locations. Researchers at the National University of Singapore are developing cultivation methods that can be carried out on seawalls, which cover nearly 80% of Singapore’s coastline and are relatively low in species diversity and productivity compared with the natural shorelines they have replaced.

By converting homogenous seawall habitats into farming opportunities, the hope is to expand the utility and value of urban marine environments and improve urban sustainability, autonomy, and resilience. Perhaps seaweed farms might even become integrated into daily city life – such that they are among the various types of community gardens that urban residents can utilize and enjoy.