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Israel: Marine Biology

Measuring Ultra-Low Nutrient Concentrations in the Mediterranean

| Editor: Alexander Stark

During a research cruise, it is necessary to analyse samples within a day to avoid changes in concentration due to preservation procedures.
During a research cruise, it is necessary to analyse samples within a day to avoid changes in concentration due to preservation procedures. (Source: Seal Analytical)

Researchers at Haifa University’s Marine Biological Station in Israel are exploiting the ultra-low detection limits of advanced laboratory equipment to measure extremely low nutrient concentrations in marine water.

Haifa/Israel — The scientists work in the Eastern Mediterranean which has the lowest regional concentration of dissolved nutrients anywhere in the global ocean. They therefore utilize an automated segmented flow analyzer from Seal Analytical, which has been specially adapted to accommodate ultra-low measurements. The measurement data are being used to determine the season nutrient cycling in the system, which will then be used to help understand the nature of the food web and the effects of global environmental and climate change.

The eastern Mediterranean Sea (EMS) has an almost unique water circulation. The surface waters (0-200 m) flow into the Mediterranean through the Straits of Gibraltar and from there into the EMS at the Straits of Sicily. As the water flows towards the east it becomes increasingly saline and hence denser. When it reaches the coast of Turkey in winter it also cools and then flows back out of the Mediterranean under the surface waters to Sicily, and then eventually through the Straits of Gibraltar to the North Atlantic. This outflowing layer exists between 200 m and 500 m depth.

Phytoplankton grow in the surface waters (0-200 m) because that is the only layer with sufficient light. This layer receives nutrients from the adjacent land, from rivers and wastewater discharges, and also from aerosols in the atmosphere. These nutrients are utilized by the plankton as they photosynthesize. When the plants die (or are eaten) their remains drop into the lower layer and are jetted out of the EMS. Because the water flows are so fast (it takes just eight years for the entire surface layers of the EMS to be replaced), these nutrient rich intermediate waters rapidly expel nutrients from the basin. The result is very low nutrient concentrations and very low numbers of phytoplankton – some of the lowest values anywhere in the world.

According to the University’s Prof. Krom, the maximum levels of nutrients measured in the EMS are 250 nM phosphate, 6 uM nitrate and 6-12 uM silicate. Ammonia is often in the low nanomolar range. By contrast, in the North Atlantic, values are 1,000 nM phosphate, 16 uM nitrate and 20 uM silicate, and the levels in the North Pacific are even higher.

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Valuable Data

The low levels of plankton caused by low nutrient levels, result in a low biomass of fish. Nevertheless, coastal areas generally support more fish than offshore, so the research will seek to quantify and understand the nutrient cycle in the coastal regions, which is poorly understood at present. The scientists plan to develop understandings which will inform stakeholders such as government. For example, there is a discussion about the potential for fish farms off the Israeli coast, so the work would enable science-based decisions regarding the quantity of fish that the system can support, Krom says.

To-date, three data sets have been taken from the EMS, and the first publishable paper is in the process of being prepared.

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