Spatial Variability In Marine Animal Migration Research.

Marine

A good knowledge of spatial isotopic variations in the environment is a fundamental prerequisite for the effective use of isotopes as a tool in animal migration research, however there are currently few accessible isoscape reference maps available for marine predator. Brault, et al (2018) conducted a study investigating Carbon and Nitrogen zooplankton isoscapes in west Antarctica - a both temporally and spatially dynamic system which is being significantly altered by climate change. "Warming is predicted to result in increased upper ocean stratification and altered phytoplankton assemblages with unknown long-term consequences". In marine systems, isoscape gradients at the base of the food web have been described at very large spatial scales, with few regional maps of marine isoscapes available, making this a main limitation of the isotopic method.

Isoscapes generated from organisms at the base of the food chain (such as the zooplankton studied by Brault, et al 2018), provide an integrated view of the spatial gradients of the whole system. Fish otoliths are almost completely mineralized from the carbonate of the water in their environment, therefore by by comparison of the water and of an individual's otolith the location of the formation of that fishes otolith can be revealed. Hydrogen isotopes are commonly used in the study of migratory birds, however as the marine environment is spatially homogeneous in regards to hydrogen they are not a plausible measure to assign marine animals to a likely geographic origin, carbon and nitrogen are much more useful in this environment as their content varies spatially within phytoplankton which is passed through the food chain and can be used to infer animal movement patterns across marine isotopic gradients. To understand these unknown long term consequences studies like this are vital to enable us to understand our current position and those of our migratory animals and so we can track changes in the future and monitor global patterns.

Aims

The applications of isoscapes as a tool in animal migration research is often limited by the existing geospatial isotope information available. The Southern Ocean was an area lacking such information. Brault, et al (2018), conducted a study to correct this and comply the first empirically derived isoscopes from zooplankton for West Antarctica. When we take the rapid chemical, physical and biological changes that Antarctica is currently experiencing as a result of climate change, it is critical that we understand the biogeochemical cycles underlying these food webs and ultimately the ecological response to these changes. This particular area was chosen in account of the major role it plays in the lives of seabirds, fish and marine mammals as well as from an economic perspective, its role in fisheries. Torniainen (2017) carried out a study in the Baltic Sea investigating spatial variability on O and dissolved inorganic C, alongside demonstrating how these isoscapes can “reveal locations of individual animals via spatial probability surface maps using the high-resolution salmon otolith isotope data from salmon during their sea-feeding phase in the Baltic Sea”.

Methods

Brault, et al (2018) analysed carbon and nitrogen isotope values (δ15N and δ13C) in 94 specimens of zooplankton from 4 locations in West Antarctica. Estimates of foraging habitats from C values in top predators is often obscured due to a combination of a lack of knowledge of basal isoscapes in the relevant marine areas and a difference in values between the base and apex of a given trophic web. “This isotopic difference results from the successive small 13C enrichment occuring at each trophic level and from various intrinsic and extrinsic factors that can affect the enrichment factors of different organisms and tissues all along the food web”. Previous investigations on the long-term movements of individual Atlantic salmon have been conducted using extrinsic tags, however due to the size of these markers they could only be attached to large individuals therefore not giving a representative picture of the population as a whole. Torniainen et al (2017) therefore set out to use isoscapes as an alternative more representative approach to track these animals over large geographical distances. This also prevented the extremely difficult task of having to capture and recapture individuals as with this approach every animal on the planet is marked by their environment and by their diets.

Results

MacKenzie et al, (2014) states that “to date, the limited number of detailed and spatially explicit, regional and in situ sample based marine isoscapes has limited the application of the techniques in the marine environment”. The study by Brault, et al (2018) was a crucial first step in the establishment of spatial isotopic patterns in Antarctica and will be valuable in addressing an array of ecosystem questions as well as a potential tool in animal migration research. The area of the Southern Ocean and our ability to understand it is one of great importance as this vast area is not only one of the largest but is one of the most dynamic ecosystems on Earth, currently experiencing rapid climate change and playing a vital role in the globes climate, fisheries, biodiversity and biogeochemical cycling. Brault, et al (2018), found strong gradients in carbon and nitrogen isotopic values in zooplankton across the 5 regions, driven by oceanography, biochemistry and productivity. Interestingly they found no such geospatial gradients in the isotopic values of phytoplankton in the same regions - they attributed this to the quick turnovers and highly dynamic nature of phytoplankton.

In conclusion to their study Brault, et al (2018), state the importance of filling the gaps in data which are needed in order for this technology to reach its full potential as a tool in animal migration research. In addition they conclude that there is a need to examine temporal variability "in isotopic baselines as a result of physical conditions fluctuating across seasons and climate modes".

Torniainen et al (2017) found that there was both a clear latitudinal and vertical gradient for both O and C, however seasonal O differences between summer and winter were only slight whereas these varied substantially for C due to algal productivity. Jaegar et al (2010) concluded their study by stating that it was their opinion that describing isoscapes using top predators themselves would be the best way to interpret the C signature of top predators.