For many years now, questions have arose concerning the role in which marine mammal faeces contributes to surface productivity of a stratified ocean, this article hopes to contribute to the existing knowledge of marine mammals possibly playing a much bigger role within nutrient cycling than once thought.

Figure 1: A model designed to represent ‘the whale pump’. Zooplankton feeding at the surface and expelling faecal pellets that sink and are lost to the bottom. Whales feed at depth and are mostly tethered to the surface for respiration. This is where excretion for these animals takes place and so, contribute to nutrient cycling. Source

Introduction
A body of water can become stratified when various factors (i.e. Salinity, temperature, pressure, etc) becomes too dense and, in turn, any mixing of water comes to a standstill. The presence of this ‘barrier’ being formed means that production is likely going to be effected because of the limited availability of nutrients reaching the euphotic zone.

The amount of nutrients that are no longer present can cause declines in phytoplankton abundance because in order to survive, access to four nutrients is required. These nutrients are: Phosphorous, Nitrogen, Carbon and Iron, and when the nutrients are unable to reach the surface then this can start to be detrimental to their survival.

Nutrient cycling is normally caused by a physical process such as upwelling of cold, nutrient-rich water into the euphotic zone for the phytoplankton to utilise. But, during periods of stratification, where the mixing is unable to occur then nutrient cycling is still present, but in a much different way. A biological process that is termed ‘the whale pump’ (Figure 1).

 

The whale pump
Originally, it was thought that only fish and copepods, whilst displaying diurnal migrations, were responsible for nutrient cycling, carrying nitrogen back up to the surface. However, within the last 7 years, evidence has came to light showing that this is not true. Marine mammals such as Cetaeceans and Pinnipeds have been found to be a much larger cause of iron and nitrogen recycling, that is seen in the euphotic zone.

So much so that with all of the whales and seals from the Gulf of Maine combined, could be held accountable for the 2.3×104 metric tonnes of nitrogen being pumped into the euphotic zone of the gulf each year. As stated earlier, copepods and small fish all excrete faecal matter that sinks, thus meaning they contribute to the transport of nutrients further down into the water column, creating a downward biological pump. It is this knowledge of a downward pump that led to assumptions of marine mammals also producing faeces that is lost to the benthos too. However, this expectation neglects the fact that whales and seals have to resurface as they are air-breathing organisms, and so must contribute in a different way (Figure 1).

Marine mammals will often dive to various depths in order to find food resources, and as they rise to surface it is likely that they carry nutrients via excreting faeces into the water column, allieviating the amount of nutrients available. It is found that marine mammals faeces does not sink, and instead it is released into the surrounding waters as liquid plumes, containing large amounts of nutrients that float and likely used up by the phytoplankton. These faecal plumes that disperse in the top layers of the water column, reverse the previous expectation of a downward biological pump that was once thought (Figure 2).

Aside from the plumes of faeces being excreted, another body of evidence further supports marine mammals boosting marine productivity. Whilst these organisms exhibit diving behaviour, they have a reduced metabolic rate at depth, and so excretion of any faecal matter is likely to occur at the surface when respiring and metabolic rates return to normal.

Figure 2: A sperm whale releasing a plume of liquid faecal matter that disperses across the euphotic zone rather than sinking to the benthos. The plumes are nutrient-rich and are an excellent source of nutrient cycling.

 

Do we really need whales then?

Most definitely. Pershing, et al., (2010) explored the idea that if whales went extinct, would we see any drastic changes to marine production? The answer is yes. The study found that there was a correlation between the whaling industry and the amount of carbon export in the years that they started to decline. For example, the blue whale (Balaenoptera musculus) numbers declined from a population of 340,280 to 4,727 and 762,400 down to 109,600 for the fin whale (Balaenoptera physalus). The amount of carbon export per year for both species was close to hitting 100,000 tonnes per year pre-whaling, but since their numbers declined, the study showed a decline to below 10,000 tonnes of carbon export.

It is true that the whaling industry has been drastically ceased in recent years, but unfortunately the whale populations have not recovered and biomass from these organisms is still less than 25% than it was before whaling began.

Due to anthropogenic activities taking place, it caused significant impacts to biomass, growth and populations of whales and seals, because larger individuals were more favoured when whaling started. This meant that the desire for large organisms to be culled, the oceans ability to sequester carbon was impacted greatly. Due to phytoplankton populations being much greater in comparison to that of whales and other marine mammals, it means that their role within nutrient cycling is overlooked once again.

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