Today Cristina continues her fascinating analysis of our Sea Trust Stena Europe survey data-sets...
Post 4. Ferry surveys – Under the sea...
Borada ffrindiau!
Continuing with our ferry surveys saga, today we will be talking about depth! This week’s post won’t be as long as usual. Instead I have been focusing in generating some visual material, so you can have an even more realistic (?) idea of what the seabed looks like across the Irish Sea and Celtic sea
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Image 1: Celtic Sea/Irish Sea benthos (Seabed).
Continuing with our ferry surveys saga, today we will be talking about depth! This week’s post won’t be as long as usual. Instead I have been focusing in generating some visual material, so you can have an even more realistic (?) idea of what the seabed looks like across the Irish Sea and Celtic sea
.
Image 1: Celtic Sea/Irish Sea benthos (Seabed).
The Fishguard-Rosslare transect presents depths that range between 20 to 120 meters approximately. If we consider that the average depth of the Atlantic Ocean is around 3300 meters and the maximum registered depth of any ocean, the Pacific is 10994 meters (The Challenger Deep in the Mariana Trench),
So our maximum transect depth of 120 meters is like the under-2-year-old's swimming pool of the oceans.
However, many interesting things occur within these ranges of depth, (also known as the Epipelagic zone), which extends from the surface to a depth of around 200 meters. This is the zone where most of the suns light can penetrate.Sunlight is a key aspect, as photosynthesis can take place at these depths. Phytoplankton, algae and photosynthetic organisms produce oxygen (actually, it is estimated that they make up for between 50% to 85% of the oxygen in the atmosphere !). The zooplankton feed on these organisms and at the same time zooplankton becomes food for larger animals fish and seabirds, and so the food chain keeps growing attracting the ultimate consumers at the top of the food chain such as whales, dolphins porpoises and sharks.. A combination of factors such as day length, temperature, waves, wind, and salinity, create this fiesta of biodiversity within this epipelagic zone! And by happy chance, it turns out, we can find ourselves slap bang in the middle of this fiesta when we doing our surveys on the Stena Europe!
So our maximum transect depth of 120 meters is like the under-2-year-old's swimming pool of the oceans.
However, many interesting things occur within these ranges of depth, (also known as the Epipelagic zone), which extends from the surface to a depth of around 200 meters. This is the zone where most of the suns light can penetrate.Sunlight is a key aspect, as photosynthesis can take place at these depths. Phytoplankton, algae and photosynthetic organisms produce oxygen (actually, it is estimated that they make up for between 50% to 85% of the oxygen in the atmosphere !). The zooplankton feed on these organisms and at the same time zooplankton becomes food for larger animals fish and seabirds, and so the food chain keeps growing attracting the ultimate consumers at the top of the food chain such as whales, dolphins porpoises and sharks.. A combination of factors such as day length, temperature, waves, wind, and salinity, create this fiesta of biodiversity within this epipelagic zone! And by happy chance, it turns out, we can find ourselves slap bang in the middle of this fiesta when we doing our surveys on the Stena Europe!
Image 2: Benthic map relating to Ferry route/transect.
Last week we introduced some heat-maps that allowed us to quickly visualize where the main hots-pots of sightings for Harbour porpoises and Common dolphins were recorded. And we also analysed how these sightings were correlated to the distance to the coast. Using the same approach, that is, selecting a random subs-sample (May to August from 2009 to 2011*), we have generated these maps again, but this time we have also include the Risso´s dolphins and Minke whales
Image 3: Species heat-maps.
Since we have been focusing on Minke whales and Risso´s dolphins as well, you may have wondered why we didn´t also include some heat-map examples for these two species last week. The reason is that last week we focused on how the distance to/from the coast helps us understand the distribution of the species. And once again, we see the same pattern, the number of porpoises’ sightings increase as we get closer to the coast, while the common dolphins´ sightings diminishes.
Now, interestingly, when we look at the Minke's and Risso´s heat-map, the distance to the coast that we discussed last week, doesn´t seem to fully explain their distribution. Although there seems to be a preference for offshore areas, there is also a predilection for western areas. Following the same approach as in previous posts, we can visualize the distribution of sightings by longitude.).
Now, interestingly, when we look at the Minke's and Risso´s heat-map, the distance to the coast that we discussed last week, doesn´t seem to fully explain their distribution. Although there seems to be a preference for offshore areas, there is also a predilection for western areas. Following the same approach as in previous posts, we can visualize the distribution of sightings by longitude.).
Image 4: Sightings Vs Longitude diagram.
Since we now have an idea of what the seabed looks like, we can compare these graphs with the depth profile of the transect.
Image 5: Transect Depth Profile diagram.
We can now see, how the depth also plays a key role in the distribution of these species, but it also leads us to next questions: why do we have a different number of sightings for a species, such as the Minke whale, for the same depth or for the same distance to the coast? Why the number of porpoise sightings seems to be higher in the Welsh coast? Does the slope of the seabed also influence this distribution?
As always, we always welcome your ideas and inputs!
Hasta la vista! Cristina.
Hasta la vista! Cristina.
* perhaps we should also look at seasonality (Cliff?)
BTW, if you want to know more about the ocean zones and the interactions between the organisms let us know, we can write about it in future posts! I also came across this site some time ago and I highly recommend checking it out! https://neal.fun/deep-sea/