There’s plenty more fish in the sea…?

This episode in the ‘searies’ we are, true to Little Mermaid’s Sebastian, zoning in on life under the sea. Our rich, colourful marine ecosystems, still in areas an enigma to us in our knowledge and understanding. So far in the searies we’ve covered how the oceans themselves function; how they move, what they’re made of,  how they influence weather patterns and events; and of course how these are changing (I won’t repeat these so take a quick gander at the last blog if you haven’t yet!). 

So, this blue home to many is altering, and fast. The Intergovernmental Panel for Climate Change forecasted that it will warm by between 2-4°C by 2100. But what does that rapid change mean for aquatic life? From readings of several papers and reports, I see this in 3 categories: 1) The species themselves 2) their habitats and 3) the larger ecosystemic impacts. Not all these changes will be felt the same across species and so, just like with any change, some will experience greater wins and losses which will then ripple out to the ecosystem they are part of. I stress that I won’t be mentioning all of these changes (phew I hear you say, I thought this was a light bite read) instead a ‘sample platter’ of some well agreed impacts in the scientific community (but apologies as this course is more substantial than previous blogs). I hope this piece gives you an increased affinity for the diverse species, interactions and lifecycles within our seas, whilst also recognising their fragility. 

1. The Species

Given changing conditions in their immediate environments, organisms can potentially both alter within a lifetime and across a lifetime, but we’ll largely focus on within a lifetime for this part.

phenotype plasticity – adaptation within a lifetime

 I know, I gave the same wince when I read ‘plasticity’ for the first time in relation to the ocean. But this is a positive, defined by a species rapidly changing behaviour, structure and physiology to a new environment in order to increase success rate. The level of capability they have of doing so dictates their ‘plasticity’. An analogy here could be a person moving to a different country, acclimatising by changing their sleep patterns to the different day-night timings, or deeper hormonal changes in response to more or less sunlight. It can allow an organism to master a novel environment.

However, plasticity needs at least some time to take place, and is determined largely by the DNA structures of the species itself. Rule of thumb is, the more complex in structure, the less plasticity. In other words, more sophisticated organisms (and these tend towards the bigger, badder prey types) will not be as readily able to adapt to the new conditions and so suffer higher losses of life in comparison. 

The other general negatives most scientific studies point towards are as follows and also lead to biodiversity loss in response to these environmental stressors:

• increased metabolic rates. Due to higher ocean temperatures, physiological activity e.g. enzyme reactions, respiration, growth, occur at a higher speed for marine life. Therefore species will need to eat more to counter this increase (hold this thought for later!)
• life-cycle mistimings. Often, a species will take seasonal changes as cues to get cracking with key life events, like development, migrations, laying eggs, hibernations. However, oceanic changes may lead to timing ‘mismatches’. For instance, coral embryos have been found to detach earlier from their colonies before they have reached maturation in hotter, more acidic conditions (Liberman et. al, 2021).
• Lower reproductive success. The above mistimings are one factor, but there are several others at play here too. Less ‘healthy’ marine organisms will have lower larvae fall, reduction in sufficient habitats for nesting may reduce success of early lifecycle development. There are even studies showing how some species reproduce much more of one sex in warmer conditions, as is the case in marine turtles becoming more feminized (Hawkes et. al, 2021).

Species response to acidity

As PH levels lower from an ever increasing absorption of CO2, this means materials are more easily dissolved in these acidic waters. What is the downside of this? Oceans will become great feasters on calcium carbonate, a compound essential in the make up of shells. So, crustacean species heavily reliant on this element may increasingly be eaten away at by the sea, evidence already showing this in certain species of sea snails. If that’s not a tragic thought in itself, this will have impacts felt across the food chain as we’ll see later. 

This second point is more well known to us, but perhaps not the ‘why’ of it. Coral bleaching is already endemic – the IPCC special report concluded that should we face a hypothetical +2°C warming scenario, a projected >99% of all coral reefs will die. But bleaching isn’t a direct result of acid corroding it as many believe, but rather warmer temperatures. When coral gets too warm, it expels the ‘colour giving’ algae living within it, turning the coral white. Since coral relies on the algae as its main food source, when bleached it struggles to maintain nourishment and is more vulnerable to disease. So bleaching doesn’t immediately spell death, but the chances are…not great.

2. Marine life habitat

The great immigration. It’s a certainty that marine life will move to different locations in order to overcome changing conditions. Largely this will involve moving away from the tropics and more towards the poles as an escape from the increasing temperatures. But let’s dive in deeper to some of the causes and consequences of this marine movement:

1. Nutrients depletion. Yes, for many, food will be in short supply in their previous habitats, and given ecosystems are geared towards feeding procedures, whole shifts are expected in search of the stuff. Combine this with the higher metabolic rates covered above, and the search is even moreso important. As ocean currents are changing, this causes sea nutrients to be less bountiful in previously abundant areas as well as phytoplankton that form the basis of many marine food chains. So the ripples upwards is thus: predators following prey following the new ‘hotspots’ for plant nutrients (see part 3 for the macro effects here)
2. Deoxygenation. Given oxygen levels are so depleted in areas largely as a consequence of human pollutants, this creates ‘hypoxic’ conditions that cannot sustain life, literally known as ‘dead zones’. The biggest dead zone in the US emerges every Spring in the gulf of Mexico, growing to 6900 square miles in 2019, or 640 acres(!) These zones lead to death and displacement of species to other locations.
3. Marine heatwaves. Defined as prolonged periods of very warm sea surface temperatures lasting weeks to months, findings have shown climate change has dramatically increased their frequency (the last 8 out of 10 major ones took place after 2010). Those most vulnerable to these events? ‘Foundation species’ – kelp, corals, sea grass. So the devastation of a heatwave can knock out that vital bottom layer of the chain. Those further up the chain will then hunt to replace that missing link needed not just for nutrition, but for nursing, predator hiding places and more.

So, these aspects will cause either abrupt or gradual movement of species, looking for food, shelter, places they can actually breathe. It’s important to know the short term implications of this may falsely look ‘good’, after all biodiversity will increase in certain locations. Of course, it only takes time for the actual consequences to set in, as we look at the larger system effects…

3. Ecosystemic changes

The Encyclopedia of Biodiversity defines a marine ecosystem as ‘communities of organisms and their physical, chemical, and geological environment – distinct assemblages of species coevolved with a particular environment over long periods of evolutionary history’ with ocean ecosystems as having longer ‘evolutionary history that preceded the colonization of land’ (Grassle, J.F., 2013). So these intricate systems have evolved over millenia, hence rapid alterations to their environment can have large impacts on their delicate balances.

The size spectrum will change. The size spectrum generally refers to the distribution of organism size-frequency within a particular ecosystem (Heneghan et. al, 2019). Imagine this looking like a sort of pyramid for a standard ecosystem:  there are larger numbers of ‘small organisms’ beginning with photosynthesis, then middle numbers of prey and fewer numbers of predators as they go up the food chain levels (these called trophic levels). This provides a sort of balance as energy is transferred upwards through each level. But with climate change effects in the ocean this energy transfer may well become less efficient, and so felt most acutely by those at the top. But why this inefficiency?

Trophic inefficiency from immigration. As species move to follow their nutrient sources, they will adapt at different rates. This means for some not having the ideal prey choice at their ‘fin-tips’ (sorry, couldn’t help myself). The result is a less nutritious meal for organism A, and given organism A is organism B’s meal, and so on and so forth, less and less nutrition makes its way up that pyramid. Again, let’s remember they will have to eat more too given an increased metabolism, these two changes together make the problem worse. Also, given deoxygenation, this also spells higher difficulties surviving for the larger sized aquatic life. Oh dear, in this instance, size really does matter.

Extinction debt. Remember I mentioned there will be a short term biodiversity increase measured in some locations so the untrained eye may see a positive? Well, this will likely be a ‘debt’ caused by a time delay between a given ecosystem stressor (loss of habitat, trophic inefficiencies etc.) and the consequence of biodiversity loss as species struggle to survive in those ‘credit’ conditions. Think of it like a person taking out loans, increasingly struggling to pay these off without access to earnings so they take out more and more credit. Eventually, they will go ‘bankrupt’, or extinct in this case. So, for a while it will be difficult to really quantify the overall biodiversity losses caused by these ecosystem shifts, as species move to new habitats that will look like they are abundant with marine life to begin with. Studies found these time gaps ‘have been sustained varies from 5 to 570 yr, and projections of the total period required to settle a debt can extend to 1000 yrs’ (Figuierdo et. al, 2019). Irrespective of how soon bankruptcy looms, overall there is a clear downwards debt spiral.

Wrapping up…

The topic of marine life adaptation to oceanic changes is frequently discussed. Could species adapt to changes in current? Will they be able to survive if they move to new locations and alter their feeding or lifecycle practices? I don’t doubt that the answer to these questions would be an enthusiastic ‘yes’…if the rate of change was welcoming of adaptation, either within a species lifetime (plasticity) or through its evolution. But the rapid rate of warming, acidification, deoxygenation and more combines to form a huge watery hurdle to overcome in that process of adaptation. Marine species depend on detailed interactions both with their habitats and each other, and most will know that outward dependencies of anything make change harder to master (thank you agile theory!).

I want to make clear that there are huge efforts globally being made to conserve species and habitats, with monitoring technologies getting more sophisticated to be able to help demonstrate effective conservation measures. For example, the gulf of Mexico dead zone task force initiated the Northern Gulf of Mexico Ecosystems & Hypoxia Assessment (NGOMEX) in 2000 to fund multiple annual research projects and recommend preventative measures. But these efforts, however large, may be a literal drop in the ocean to the rising annual greenhouse gas emission figures and their destruction of marine life. We must give the gift of time to ecosystems and that starts with stopping emissions.

References:

• Patricio, A.R., Hawkes, L.A., Monsinjon, J.R., Godley, B.J., Fuentes, M.M.P.B., ‘Climate change and marine turtles: recent advances and future directions’, Endangered Species Research 44 (2021), available here. Last accessed 30/11/22
• Shastri, A., ‘‘Increasingly Acidic Oceans are Dissolving Sea Snails,’ Science in the News, Harvard University. Last accessed 30/11/27.
• Anon, ‘What is coral bleaching’, NOAA (2021), available here. Last accessed 30/11/22.
• ‘Summary for policymakers’ in Pörtner, H.O., D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.) IPCC special report on the ocean and cryosphere in a changing climate (IPCC, 2019). 
• Anon, ‘Hypoxia’, NOAA, (2021), available here. Last accessed 30/11/22.
• Grassle, J.F., ‘Marine Ecosystems’, Encyclopedia of Biodiversity, 2 (2013) pp.45-55. Available here. Last accessed 30/11/22.
• Anon., ‘Northern Gulf of Mexico Ecosystems and Hypoxia Assessment’, NOAA, available here. Last accessed 1/12/22.
• Hillebrand, H., T. Brey, J. Gutt, W. Hagen, K. Metfies, B. Meyer and A. Lewandowska ‘Climate change: Warming impacts on marine diversity’ in Salomon, M. and T. Markus (eds) Handbook on Marine Environment Protection. (Cham: Springer, 2018).
• Anderson, J, J.,  Gurarie, E., Bracis, C., Burke, B., Laidre, K.L., ‘Modeling climate change impacts on phenology and population dynamics of migratory marine species’, Ecological Modelling (2013) pp.83-97
• Heneghan, R.F., I.A. Hatton and E.D. Galbraith ‘Climate change impacts on marine ecosystems through the lens of the size spectrum’, Emerging Topics in Life Sciences 3(2) 2019, pp.233–243.
• Doney, S., ‘Climate change impacts on marine ecosystems’, Annual review of marine science,  4 (2012). 
• Fuiguiredo, J., Krauss, J., Steffan-Dewenter, I., Cabral, J.S., ‘Understanding extinction debts: spatio–temporal scales, mechanisms and a roadmap for future research’, Ecography: A journal of Space and time in ecology, (2019), available here. Last accessed 1/12/22.