From the last installments of Bad HABits, we’ve established that HABs are typically caused by excessive nutrient inputs as well as the qualities that make a ‘Harmful’ Algal Bloom (HAB), harmful and the potential impacts of HABs.
Certainly, it would seem that the solution is simply reduce excessive nutrient inputs in agriculture.
While reasonable in some cases, it can be challenging to determine what amount of nutrients can be considered excessive due to how variable interactions between nutrients, plants and the environment can be in different contexts. Due to factors ranging from the hydrology of an area affecting how excess nutrients are mobilised to different nutrient uptake and utilisation rates by plants even between members of the species, which can be influenced by parameters such as pH, to even the cultural expectations that fertiliser be used regardless of soil fertility.
Even putting aside how difficult it would be in practical terms to conduct a comprehensive evaluation of each plot of farmland as mentioned above. It is important to understand that synthetic fertilisers have allowed developing nations to build up their economies and keep up with growing demand. And while it is a worthwhile goal, it takes a certain type of privilege for us as a developed nation to say we should cut fertiliser use to reduce the environmental impact of agriculture. It is a complex issue and the big picture appears dire; large-scale fertiliser use is a consequence of having to feed our world, yet it leaves us stuck in a bind where HABs become more frequent and prevalent threat.
Without reducing nutrient inputs in water, are we helpless against the HAB scourge?
Not exactly.
From Bad HABits part 3: “However, macroalgae in an ecosystem may actually mitigate the effects of eutrophication by taking up excess nitrates and phosphates (spoiler alert: this will be important in later posts 😉).”
From previous posts we’ve learnt that algae require nutrients similar to plants, which is why excess nutrients are so dangerous. However, should also recall that algae are an extremely diverse group. Represented by both the microalgae that we see in red-tides as well as the giant kelp that characterise kelp-forest habitats. It is this key idea that may hold our salvation (figure 1), or to be less dramatic: it will at least show one plausible solution for mitigating the issue.
The ability of seaweed to mitigate eutrophication has not gone unnoticed, in recent studies conducted in China, it is estimated that a single hectare of seaweed aquaculture removes the equivalent nutrient inputs that enter an area of 17.8 ha for nitrogen and 126.7 ha for phosphorus. Removing approximately 75,000 tonnes of nitrogen and 9,500 tonnes of phosphorus annually from Chinese coastal waters (Xiao et al., 2017). Compared to controls, areas cultivating seaweed exhibited reductions of inorganic nutrients such as ammonia nitrogen (NH4-N), nitrate nitrogen (NO2-N), nitrate nitrogen (NO3-N) and phosphate phosphorus (PO4-P) by 50–94%, 42–91%, 21–38% and 42–67% respectively (He et al., 2008).
Some have even taken the idea further, combining seaweed aquaculture with other marine organisms (figure 2). Bringing the whole ecological community in a form of farming called: Integrated multitrophic aquaculture (IMTA), employing the principle of using wastes from one process as inputs for another (Ridler et al., 2019). These systems combine aquaculture of fish with seaweed and shellfish, in an attempt to emulate ecosystem processes which mediate the flow of energy through multiple trophic levels. The concept has already been developed enough to be commercially viable and in Canada is used to produce Atlantic salmon, blue mussels and kelp in a single system.
However, as with all good things, it’s not that simple. IMTA, like any form of agriculture still runs the risk of releasing excess into the environment, which in this case comes in the form of fish waste. Hence, in-depth understanding of the duality of nutrients (essential when limiting ⁄ polluting when in excess) is important to inform the engineering of IMTA systems produce nutrients in moderate amounts, allowing for partial recapture while maintaining optimal concentrations for healthy and productive ecosystems (Chopin, Cooper, Reid, Cross, & Moore, 2012).
For all its benefits, seaweed farming and IMTA approaches have their fair share of kinks and drawbacks that have to be worked out. Despite their known capacity to reduce eutrophication and HAB occurrences, seaweed farms have fallen victim to HABs themselves. In 2013, a toxic cyanobacteria (Lyngbya sp.)caused mortalities of the farmed seaweed Eucheuma denticulatum in Zanzibar, along with health problems to the farmers including skin irritations, lesions and wounds, itching and redness of the eyes (Msuya, 2013). Msuya (2013) also reported that other seaweeds may also try to muscle in on cultivated areas, in the South-West Coast of Zanzibar, macroalga (Sarconema sp.) had covered the farmed E. denticulatum, killing them. The macroalga was in turn covered by films of microalgae. Reading this paper made me think of a battle-royale between algae.
While no solution at this point is perfect, research into IMTA is still an active topic and gives some measure of hope in the face of the uphill battle of tackling nutrient pollution. In our next topic we will briefly explore a case study in HABs, to find out what happened and how we can learn from it!
References
Chopin, T., Cooper, J. A., Reid, G., Cross, S., & Moore, C. (2012). Open-water integrated multi-trophic aquaculture: Environmental biomitigation and economic diversification of fed aquaculture by extractive aquaculture. Reviews in Aquaculture, 4(4), 209–220. https://doi.org/10.1111/j.1753-5131.2012.01074.x
He, P., Xu, S., Zhang, H., Wen, S., Dai, Y., Lin, S., & Yarish, C. (2008). Bioremediation efficiency in the removal of dissolved inorganic nutrients by the red seaweed, Porphyra yezoensis, cultivated in the open sea. Water Research, 42(4–5), 1281–1289. https://doi.org/10.1016/j.watres.2007.09.023
Msuya, F. E. (2013). Recent occurrence of algal blooms affecting seaweed farms in Zanzibar: a sign of climate change impact? Annals of Library and Information Studies (ALIS), 27(December), 90–95.
Ridler, N., Barrington, K., Robinson, B., Wowchuk, M., Chopin, T., Robinson, S., … Boyne-Travis, S. (2019). Enhanching sustainability with integrated multitrophic aquaculture Social sustainability. Retrieved from https://www.aquaculturealliance.org/advocate/enhanching-sustainability-with-integrated-multitrophic-aquaculture/
Xiao, X., Agusti, S., Lin, F., Li, K., Pan, Y., Yu, Y., … Duarte, C. M. (2017). Nutrient removal from Chinese coastal waters by large-scale seaweed aquaculture. Scientific Reports, 7(March), 1–6. https://doi.org/10.1038/srep46613