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Biological oceanography

Where are the Zooplankton?

Reference: Druon, J. N., Hélaouët, P., Beaugrand, G., Fromentin, J. M., Palialexis, A., & Hoepffner, N. (2019). Satellite-based indicator of zooplankton distribution for global monitoring. Scientific Reports9(1), 4732. 

There has been a lot of recent effort by scientists finding new and exciting ways to use satellite remote sensing to better understand where zooplankton can be found in the ocean. Zooplankton are small organisms in the ocean that are second in line when it comes to the food web. Phytoplankton are primary, as they are like the land plants of the ocean. They use sunlight and nutrients to make energy and grow. Then there are zooplankton, that eat the phytoplankton to grow. Zooplankton then become a food source for those higher up in the food chain ladder, like fish and whales. Check out my post from last month for an introduction to the topic.

Figure 1. Image of a copepod, a type of mesozooplankton. Image credit Uwe Kils and obtained from Wikipedia Commons at https://commons.wikimedia.org/w/index.php?search=copepod&title=Special%3ASearch&go=Go&ns0=1&ns6=1&ns12=1&ns14=1&ns100=1&ns106=1#/media/File:Copepod.jpg

Satellites are an incredible tool for being able to see large areas of the oceans and monitor these broad regions over and over again. However, what satellites don’t do well is seeing any of the small individuals living in the ocean. This makes for a challenge considering the basis of the marine food web are such small organisms! A recent study by Dr. Druon and co-authors used a combination of observations of zooplankton in the water with satellite data to model suitable habitats for zooplankton. It’s important to know where zooplankton can thrive because areas where there is a lot of phytoplankton growth leads to areas where zooplankton can feed, and ultimately links these smaller organisms to hotspots for other fish.  Not to mention, this can then become a popular place for fishing. As Druon and co-authors say,  “mesozooplankton can be considered as the keystone of marine food webs linking low and high trophic levels.” Mesozooplankton are just a category of zooplankton that range in size from 0.2 to 20 mm, which is less than an inch! Figure 1 shows an example of a type of mesozooplanton called a copepod.

The Study

Researchers used data from the continuous plankton recorder survey This has been an ongoing to study to characterize plankton types led by the Marine Biological Association in Plymouth, UK and has been in active operation collecting data for over 60 years! They put the plankton recorder on a variety of ships that tow the instrument. As the instrument passes through the surface ocean water, it uses silk bands to filter and collect the plankton, which can then be analyzed in the lab.

Figure 2. Image of a continuous plankton recorder. Image obtained on Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Sahfos_cpr.gif

The data collected from the Continuous Plankton Recorder was then matched up with satellite imagery. Since the field data has been collected for so long, they were limited by the time scale starting when the satellite was launched into orbit in 2002, so their study ranges from 2002 until 2017. They use a parameter called chlorophyll. Chlorophyll is a green pigment in phytoplankton (and also in land plants), that allows phytoplankton absorb energy from sunlight to grow and survive. Satellites can’t directly observe chlorophyll, but there are very well established and researched methods to derive chlorophyll from satellite observations. This was done through careful studies that matched measurements of chlorophyll in the water to observations from satellites that detect the light reflected back out to space. With chlorophyll observations, scientists can estimate the amount of phytoplankton present in the water at that time. This is relevant to then understanding if mesozooplankton are present in the water because these mesozooplankton feed on phytoplankton.

This study in particular is also interested in looking at the gradient in chlorophyll, meaning how the value of chlorophyll changes in space. They use this as a metric for productivity fronts, a region that is high in chorophyll and thus high in primary productivity.  The data from the satellite-derived chlorophyll and chlorophyll gradient was used as input into a model, which they called the habitat model. While I won’t talk specifically here about the details on this analysis, the main goal of modeling is to predict areas where mesozooplankton would likely have enough prey (phytoplankton) to survive.

Results

The Continuous Plankton Recorder observed 54,282 samples between 2002 and 2016, which is an enormous amount of data considering it is collected in the field! The time of year when most of the mesozooplankton were found was during the spring and summer months and they found most of their zooplankton off continental shelves in the north. The continental shelf is the region where the land mass extends underneath the water so the water is still relatively shallow.

The mesozooplankton environments were characterized into four different types based on how the data from the Continuous Plankton recorder matched the chlorophyll and chlorophyll gradient data from satellites. The analysis revealed that almost half of their observations fell into a category that had medium-high levels of biomass, where biomass refers to the amount of mesozooplankton present. This category of medium-high levels of biomass occurred most frequently in autumn. The least percentage of their data, only 4%, fell into a category that was considered to have high amounts of biomass. Approximately a quarter of the data had very low levels of biomass and occurred either in the northern area of the study during the winter months or further south in the study region. These results gave the researchers a sense of where the mesozooplankton are actually living and how it relates to what they could see from the satellite.

From the suitable habitat model, researchers found that in the spring and summer months there was a much larger area of suitable habitat in the northern study region. However during the winter there were fewer occurrences of suitable habitats and the locations of these habitats were confined to a smaller space surrounding the land.

Like all studies, it is important to understand areas that research is limited and could be improved in the future. For example, using satellites as monitoring tools are limited by cloudiness and ice cover. Satellites can also only detect biological signals from the surface water, so any organisms living below the surface will be undetected. The researchers suggested that these limitations likely could underestimate their predictions of where mesozooplankton would inhabit.

Big Picture

Overall, this study offers a new way to use satellite data to understand potential areas of mesozooplankton habitats. Authors suggest that their work could help monitor the ocean habitats in the future. Monitoring ocean habitats are critical with climate change because we need to understand how the organisms in the ocean are responding to change. 

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