What impacts can we expect from the expansion of low-oxygen zones in today’s ocean?
An international team of scientists that includes Michigan State University paleo-oceanographer Dalton Hardisty has received a three-year, $1 million grant from the National Science Foundation, or NSF, Division of Ocean Sciences to conduct research related to that question.
Together with scientists from Woods Hole Oceanographic Institution, or WHOI, and the Leibniz Institute of Baltic Sea Research in Germany, Hardisty will explore the chemical reactions and coupled cycling of manganese, iodine, nitrogen and oxygen in low-oxygen, or suboxic, zones of the Baltic Sea in northern Europe. Manganese, in particular, participates in and influences the cycling of nearly all other marine elemental cycles, but its distribution and consequences in these suboxic zones is not well known.
Suboxic zones are areas of water between the ocean’s oxygen-rich layer near the surface of the ocean, which contain sufficient oxygen to support animal life and its anoxic zone in the lower depths; where oxygen cannot be detected and only microbial life persists.
Similar to other estuaries, the intersection of freshwater and saltwater in the Baltic Sea causes stratification between distinct density layers; however, the size and submarine topography of the Baltic uniquely enhance these properties, limiting oxygen replenishment at depth and leading to well-defined anoxic and suboxic areas. In combination with these natural conditions, human contributions of nutrients from agriculture and industry further exacerbate oxygen depletion in the Baltic Sea and other similar coastal zones.
“The Baltic Sea is a eutrophic basin, partly because nitrogen from fertilizers on land stimulate algal blooms in the upper layers of the Baltic’s water column,” said Hardisty, endowed assistant professor of Global Change Processes in the MSU Department of Earth and Environmental Sciences. “These algae soon decay, consuming even more oxygen in the process, which then affects the distribution of life in these settings.”
Together with eutrophication, warming waters associated with climate change have led to an expansion of suboxic waters in both the world’s coastal and open ocean settings over the past half century. Beyond the detrimental impacts they can have on biology, these waters also host specialized chemical reactions whose pathways are not well understood, but which are increasingly important to understand as low oxygen waters become more abundant.
“The Baltic Sea is among the world’s largest low oxygen estuaries, making it ideal for characterizing suboxic chemical reactions important across the globe,” Hardisty said.
Hardisty has visited the Baltic Sea before, using the geochemistry of sediments from drill cores to reconstruct the history of ancient seawater oxygen depletion in this region. He is now turning his sights to modern seawater, with a focus on the cycling of manganese and related elements relevant to the Baltic and beyond.
Because manganese, iodine, nitrogen and oxygen are all very chemically reactive under suboxic conditions, forming multiple different bonds with one another, studying their linked elemental cycles in the Baltic would reveal the importance of similar reactions throughout the ocean. That information would then provide insight into how the currently expanding suboxic zones in today’s evolving oceans might affect life and ocean chemistry more broadly.
To study these conditions in the Baltic, the collaborative team will conduct research expeditions on specially equipped research ships out of Germany, one trip in each of the first two years of grant funding. Experiments will involve collecting seawater in airtight containers directly from suboxic zones in 16 different geographical areas of the Baltic Sea, adding tracers of isotopes in each element and then tracking chemical reactions in these incubations, which act as natural laboratories.
Although much of the experimental work will be conducted at sea, Hardisty and his graduate students will also bring samples back to MSU to conduct more detailed chemical analyses. Their research responsibility on the collaborative team will be to explain the role of iodine in the manganese and oxygen cycles. The MSU group specializes in studying the iodine cycle and other oxygen-sensitive elements in both the modern and ancient oceans.
“The MSU graduate students leading this research will be implementing state-of-the-art techniques using liquid chromatography and inductively coupled plasma mass spectrometry to separate and quantify the abundance, chemical distribution and fate of the radioactive iodine isotope, iodine-129, during these shipboard experiments,” Hardisty said.
By coupling field measurements and targeted shipboard incubations, this study will shed light on the processes controlling the manganese cycle and its link to the oxygen, iodine and nitrogen cycles.
“I am extremely excited for the opportunity to be a part of this research and the collaboration with WHOI and the Leibniz Institute of Baltic Sea Research,” Hardisty said. “This is an emerging and fundamental topic within chemical oceanography and the combination of the study site and novel approaches specific to the research team will undoubtedly be a major step forward for our understanding of the biogeochemistry of low-oxygen settings and the implications for their expansions in the ocean.”
