Blooming Hell!

Hello again,

Today's post is going to be a little different and self indulgent; as a member of the Environmental Modelling MSc course at UCL I was particularly interested in a paper I referenced last week by Lai and Yin (2014). The paper investigated the potential for Algae Blooms to derive from physical accumulation via the application of a coastal circulation model.

Typically, algae blooms are considered to result from excess agricultural nutrients entering the aquatic system and promoting the growth of diatoms or other single celled algae to an exaggerated level (eutrophication). As the concentrations of algae increase they reach toxic levels (referred to as 'Harmful Algae Blooms' or HABs), ingestion of which can increase mortality rates of marine life and cause illness in those that eat the affected fish/shellfish etc. Backer and McGillicuddy's (2006) study on the relationship between humans and the ocean contains a plethora of illnesses that can be caused by ingestion, although I wouldn't advise reading it if you're at all partial to seafood!

Delicious! a glass of Microcystis bacteria (this is a surface sample of a HAB affected freshwater lake). These bacteria excrete neuro-toxins which can kill humans (Oberholster et al, 2004). Photo credited to T. Bridgeman in (Backer and McGillicuddy, 2006). Presumably Mr Bridgeman is no longer with us.

Lai and Yin's (2013) study theorised that, aside from nutrient pollution, blooms can arise from a combination of physical factors causing an accumulation of the single celled algae. Their theory was based on observations in Port Shelter bay in north-east Hong Kong; the bay suffers from frequent HAB's of Dinoflagellates that tend to occur in the same area each time (a band mirroring the coastline). Water samples from the bay indicate that nutrient levels are too low to produce a eutrophication event and hence a second, physical factor seemed likely.

In order to understand the complex mechanisms at play a Finite Volume Community Ocean Model (FVCOM) was used in combination with a series of water samples that were taken during a bloom event and identified Algae concentrations. The model was utilised in 3 experiments, each increasing in complexity by the application of new variables, with assessment of the potential for aggregation:

• Experiment 1 involved two model runs: Both utilised the morphology of the coastal region and the effects of the tidal forcing on the movement of algae in the bay. However, 1A was performed under a homogenous water profile (basically ignoring the effects of changing temperture and salinity levels based upon water depth), whilst the 1B was performed under a stratified water profile whereby the temperature and salinity (which both affect algae concentrations) were utilised to aid aggregation of the Dinoflagellates.
• Experiment 2 maintained the stratified water profile and tidal forcing but included the effects of wind conditions on the surface of the water.
• Experiment 3 enhanced Experiment 2 with the inclusion of the ability for the Dinoflagellates to swim upwards, against flow in order to promote photosynthesise. This ability results in amplified accumulation as the flagellates are suspended in the same location, swimming against the current, for longer periods of time.

The experiments proved that by applying several variables:

• An accurate approximation of the coastal topography
• Tidal forcing
• Wind effects
• A stratified, saline/temperature dependent water profile
• A potential for the modelled particles (in this case the Dinoflagellatte algae) to vertically migrate towards the surface

The FVCOM model can simulate the physical aggregation effects observed across Port Shelter bay! Woohoo! Modelled cross-sections across portions of the bay that have been observed to contain HAB's clearly indicate that there is a convergence of currents (and hence Dinoflagellates).

Cross sections of the modelled bay indicating convergence of currents. Larger arrows on the surface indicate position of observed HABs whilst smaller arrows indicate current direction (Lai and Ying, 2014. page 73).

Cartoon representation of the modelled process (Lai and Ying, 2014. page 74).

I hope this sortie into my preferred discipline has given you a little more insight into how we can begin to enhance our understanding of the environment. I feel that mitigation of human impacts on the environment must begin with a good understanding of how the environment works and how our impacts actually affect it. As we have seen in this post, natural processes can cause similar outcomes as those derived from humans. Without this further investigation we may have assumed that such HAB events are solely due to our influence, giving us a misguided sense of our impact.

As a thanks for joining me again, here's another Harmful Bloom.

Courtesy of lotr.wikia.com.