The Gulf of Mexico (GOM) Loop Current (LC) forms as water flows from the Caribbean Sea into the GOM through the Yucatan Peninsula. The LC intrudes north, and eventually forms eddies which shed periodically and drift to the westward, driving the deep circulation throughout the Gulf of Mexico. The LC has a complex cycle of northern excursion, detachment of eddies, with sometimes several reattachments before finally detaching from the main LC circulation before propagating westward throughout the GOM. Following LC eddy separation, the LC retreats much further south, and begins to extend north once again. Due to the strong velocity of the LC waters, a front is set up which establishes a natural barrier for planktonic organisms. Eddy shedding is occurs every 9-11 months, and it was recently established that there is a seasonality to the eddy shedding. As eddies detach and propagate westward, high salinity species which would not normally be seen in the region will likely become evident. As this high salinity, subtropical warm water masses propagate west into the GOM, they can be used as indicator species of tropical and high salinity species throughout the GOM. There may be some seasonality to the distribution of tropical species in the western GOM with the seasonal cycle of eddy detachment. Additionally, these eddies contain species from the Loop Current and can collect species from the surrounding waters as they propagate, it is possible to study the effects eddies have on species distribution and biomass variation through the GOM. Therefore, understanding the separation period of these eddies can help to explain the zooplankton community in the GOM and can be used to help describe their abundance and distribution. It could be hypothesized that with propagation of eddies into the western GOM, there would be increased diversity as these tropical species mix with the western GOM species.
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In a study in the western GOM, researchers attempted to identify whether they could identify separate zooplankton community groups based on location within anticyclonic and cyclonic eddies. They did not find that location within these groups was significant, but deduced that the communities were based more on depth than association with the LC eddies. Additional, sampling within a LC eddy as it propagates west could lead to a different result as these eddies were very far to the southwest of the GOM and have a lot of time to acclimate to the environment. With more northern penetration of the LC, the frontal region of the anticyclonic feature can interact with the continental shelf, influencing flow and pressure gradients, potentially resuspending nutrients, and making these frontal zones favorable for biological production. In a study by Cornic and Rooker (2018), with more northern excursion of the LC, there was a greater extent of highly suitable habitat for blackfin tuna, allowing for more survival during early life stages. Conversely, years of low latitude LC penetration had lower densities of blackfin.
The LC frontal region is an area of enhanced primary productivity, and therefore increased secondary consumers (zooplankton), which thrive off the increased food source in an otherwise oligotrophic region of the GOM. In years of more northern excursion of the LC, the dynamic region where the LC and Mississippi River Plume interact provides a unique environment for zooplankton which rely heavily on environmental parameters. While the enhanced primary productivity and ichthyoplankton biomass has been explicitly discussed in the previous studies noted above, zooplankton abundance and distribution has been much less studied in relation to the northern excursion of the LC.
In the marine ecosystem, environmental variables have significant influence on the relative abundance, distribution, and species diversity of planktonic organisms. In the Gulf of Mexico (GOM), the Loop Current (LC) is the primary driving process related to spatial variations in water properties and changes in phytoplankton, zooplankton and ichthyoplankton communities. Understanding the position of the LC front and depth of the LC circulation can aid in understanding the biological community in the GOM. The 17 cm SSH contour is commonly defined as the position of the LC frontal boundary or eddy front. This front acts as a natural barrier of strong vertical sheer between the warmer, high salinity, tropical water mass and the GOM open ocean environment. Oceanographic fronts are highly productive regions for planktonic communities, commonly identified as hot spots for microbial communities in otherwise oligotrophic water masses due to availability of nutrients due with the shallowing of the nutricline. Williams et. al., (2017) noted an increase in microbial plankton near the LC front, suggesting a bottom-up controlled environment. Additionally, Williams et al., (2017) identified that regions of combined coastal water and LC frontal convergence had increased heterotrophic prokaryote abundance relative to the autotrophic plankton in the surrounding water, suggesting a shift in the community due to the LC. This shift in phytoplankton likely affects the quality of food for zooplankton and therefore the community and growth. Some species of calanoid copepods, the dominant zooplankton component in the LC system, tend to feed selectively based on the size and shape of phytoplankton. Therefore, a shift in species assemblage of phytoplankton may impact herbivorous copepods (and other selective zooplankton) in oligotrophic waters along these fronts. Differences in isotopic signatures in carbon indicated food source shifts have been noted between LC and open ocean environments in the GOM zooplankton community.
Frontal zones along the LC have been found to be important habitats for several pelagic fish species, including billfishes and tunas, acting as an important spawning ground for these commercially important fish. This region of higher productivity can contribute to bottom up processes playing an important role in the lifecycles of these fishes. Zooplankton exhibits species-specific relationships to temporal and spatial variability in mesoscale features, influencing their abundance and distribution. An important component of the zooplankton community, chaetognaths, has been explicitly studied in the LC frontal region. Since chaetognaths have weak osmoregulation, they are confined to narrow salinity ranges and therefore occupy specific oceanic zones and water masses. Chaetognaths are commonly recognized as a significant trophic link between smaller marine zooplankton, such as copepods and large predators, including many commercially important fish during their larval stage. Chaetognaths are distinctively affected by hydrographic conditions, such as salinity, temperature and dissolved oxygen with species- specific horizontal and vertical distribution patterns. Some species have strong hydrological associations with water masses and are good indicators of regional oceanographic patterns.
In a study on the abundance and distribution of chaetognaths in the GOM, species specific analyses were run to look at explanatory variables on the abundance of 4 abundant chaetognath species during 2015 and 2016 in the LC region. This study found the distribution and abundance of chaetognaths in the GOM are highly influenced by physiochemical characteristics at species specific levels. Distinct species-specific preferences were observed in relation to environmental parameters, suggesting that temporal and spatial variability in mesoscale features may play an important role in partitioning chaetognath species in the GOM. The depth of the 6°C isotherm is used in this study as the depth of the lower boundary of the LC, acting as a barrier between the LC water and the bottom water. The deeper the 6°C isotherm, the deeper the LC intrusion extends into the water column. The fast moving water of the Loop Current can reach 800 to 1000m depths, drawing warm tropical, high salinity water down to about 200m.
In stable waters in the tropics and subtropics, the vertical distribution of nutrients, primary production, and chL remains relatively unchanging throughout the year due to the solar heating stabilizing the water column and separating it into two regions. The upper region is not light limited, but nutrient limited and dependent on the chemical and biological regeneration of nutrients. The bottom community is light limited but has higher abundances of nutrients. During periods of deep LC penetration where the 6°C isotherm is lower, the oligotrophic center of the LC is nutrient deplete. This directly impacts zooplankton vertical migration, as is influenced by food availability, so therefore under low food environments in the LC domain, zooplankton decreases the extent of vertical migration. Conversely, there will be enhanced primary productivity along frontal regions of the LC, allowing for more vertical migration due to the added nutrients. The LC front has been noted as regions of higher primary productivity globally, as well as an important region for zooplankton and for larval fish growth. Specifically in the GOM, the Loop Current front has been noted as a region of significant larval fish abundance in several studies. However, the mechanism driving the high abundance of fish larvae is unclear. It may be related to the species composition of the zooplankton community or aggregations of zooplankton and fish larvae in the converging zone.
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