The ocean contains numerous chemical constituents, including trace elements. Despite their minute concentrations, these elements play a role in marine processes and the planet’s overall health. Understanding them is important for comprehending ocean ecosystems.
What are Ocean Trace Elements?
Ocean trace elements are chemical elements found in seawater at extremely low concentrations, typically less than one part per million (ppm). Despite their scarcity, these elements are distinct from major ions like sodium and chloride, which make up the bulk of seawater’s salinity.
Common examples include iron (Fe), zinc (Zn), copper (Cu), cobalt (Co), and manganese (Mn). Their low concentrations make them challenging to measure accurately, often requiring specialized ultratrace sampling and analysis techniques to avoid contamination. The distribution and behavior of these elements in the ocean are highly variable, reflecting a combination of input sources, biological recycling, and removal processes.
Why are Trace Elements Crucial for Marine Life?
Trace elements serve as micronutrients that regulate ocean productivity and the structure of marine ecosystems. They are incorporated into various enzymes, acting as cofactors to facilitate biochemical reactions within organisms. Their availability directly influences the growth and metabolism of marine organisms.
Iron
Iron often limits phytoplankton growth in vast ocean areas. Phytoplankton, microscopic photosynthetic organisms forming the base of the marine food web, require iron for processes like photosynthesis and nitrogen fixation. Iron supply can directly impact the ocean’s ability to absorb atmospheric carbon dioxide, influencing global climate regulation.
Zinc
Zinc acts as a micronutrient for many enzymes involved in metabolism and overall health. It is particularly important for protein synthesis, gene expression, immune function, and reproductive health in marine organisms. In phytoplankton, zinc is often depleted in surface waters due to biological uptake, and its concentration increases with depth as organic matter decomposes. Some phytoplankton species can even substitute cadmium or cobalt for zinc in enzymes like carbonic anhydrase, which are used in carbon acquisition for photosynthesis.
Copper
Copper is an important micronutrient, required for the growth and development of aquatic plants and as a cofactor in certain enzymes. It plays a role in both photosynthetic and respiratory electron transport chains. However, copper can become toxic at higher concentrations, affecting the growth, reproduction, and survival of fish and invertebrates.
Cobalt
Cobalt is a component of vitamin B12, a coenzyme used by marine phytoplankton and other organisms for various metabolic processes. Its presence is linked to phytoplankton growth and calcification, impacting the global carbon cycle.
Manganese
Manganese is a cofactor for enzymes, involved in nutrient metabolism, antioxidant defenses, and is a component of the oxygen-evolving complex in photosystem II during photosynthesis in phytoplankton.
Sources and Sinks of Ocean Trace Elements
Trace elements enter the ocean through natural pathways.
Sources
- Atmospheric dust deposition is a significant source, particularly for elements like iron and aluminum.
- Rivers and estuaries also contribute dissolved and particulate trace elements from the weathering of rocks and soils.
- Subglacial meltwaters from ice sheets can mobilize and export trace elements to coastal polar oceans.
- Hydrothermal vents on the seafloor release fluids rich in certain trace elements, such as manganese and iron.
- Volcanic activity similarly introduces elements into the marine environment.
- Once in the water column, these elements undergo various transformations and movements.
Sinks
Natural processes also remove trace elements from the ocean. Sedimentation is a primary removal mechanism, where elements adsorb onto particles like clay minerals, organic matter, bacteria, and fecal pellets, and then settle to the seafloor. This “scavenging” process removes elements from the dissolved phase and deposits them into sediments. Biological uptake by marine organisms also removes trace elements from surface waters; when these organisms die and sink, the elements are transported to deeper waters or the seafloor, where they can be remineralized or buried.
Human Activities and Ocean Trace Elements
Human activities alter the natural balance of trace elements in the ocean, often leading to increased concentrations with detrimental effects.
Industrial Runoff
Industrial runoff, including discharges from mining operations, smelting, and manufacturing, releases various trace metals into coastal waters. Antifouling paints used on ship hulls are a significant source of copper in the marine environment. These inputs can elevate trace element concentrations in marine habitats near industrial sites.
Agricultural Practices
Agricultural practices also contribute to trace element imbalances through runoff containing fertilizers, pesticides, and animal waste. Fertilizers often contain zinc, and excess application can lead to their leaching into waterways. Pesticides and herbicides may also contain trace metal compounds that wash into the ocean, and livestock manure can introduce heavy metals. This influx can lead to eutrophication and oxygen depletion, which can alter the chemical behavior and bioavailability of trace elements.
Climate Change Impacts
Climate change further complicates the cycling of ocean trace elements.
##### Ocean Acidification
Ocean acidification, caused by increased absorption of atmospheric carbon dioxide, leads to a decrease in seawater pH and changes in carbonate chemistry. These chemical shifts can alter the solubility and bioavailability of trace elements, potentially increasing the toxicity of some while reducing the uptake of others.
##### Ocean Deoxygenation
Ocean deoxygenation, a consequence of climate change, involves a reduction in dissolved oxygen levels in seawater. This deoxygenation can affect the redox state of the ocean, which in turn influences the speciation and availability of trace metals like manganese.
Such alterations from human activities and climate change can disrupt marine ecosystems, impacting the health and productivity of marine life and the overall biogeochemical cycles of the ocean.