EMECO

Nutrient Enrichment

Sources and Impacts of Nutrient Enrichment

Nutrients occur naturally in marine ecosystems and are vital for phytoplankton production. 

Phytoplankton are microscopic ocean plants that help regulate our atmosphere and the health of the seas. Phytoplankton constitute the base of the ocean food chain, produce around half of the oxygen generated by plants on Earth, and play in key role in reducing the amount of carbon dioxide in the atmosphere via the process of photosynthesis.

Phytoplankton growth is largely dependent on the availability of the nutrients, nitrate and phosphates. In the highly productive North European Seas, seasonal mixing of the ocean during winter provides nutrient rich surface waters. However, during winter there is insufficient light for phytoplankton to flourish. In spring, the nutrient rich surface waters warm and the increase in light levels trigger a phytoplankton bloom (the spring bloom). The nutrients in warm surface water soon become insufficient to support high phytoplankton production. Consequently, the spring bloom is short lived. As the autumn begins, the warm, nutrient poor surface waters begin to mix with the cooler, nutrient rich deep waters and the autumn bloom is observed. The autumn bloom is relatively small due to limited nutrient availability and is short lived due to diminishing light intensity.

The spring bloom is crucial for microscopic animals (zooplankton) that graze on the phytoplankton. Zooplankton and phytoplankton are collectively termed plankton, and this abundance of plankton is an important seasonal food for many species of fish e.g. sandeels and herring, which, in turn are critical for the survival and reproductive success of many predators, including commercial whitefish (e.g. cod and haddock) and seabirds (e.g. puffins and guillemots).

The timing, size, and distribution of the spring bloom can reflect important changes in the marine ecosystems and threats to ocean health and is dependent on the availability of nitrates and phosphates. Nitrates and Phosphates occur naturally from sinking organic matter (e.g. faecal pellets or dead organisms). However, nutrients from agricultural, fish farming, and sewage also enter the marine environment via outfalls and rivers and act as an added source of nitrates and phosphates. Excess nutrients from pollution can lead to the process of eutrophication, which may radically change the marine community and adversely affect the systems health.

Eutrophication of marine systems can be diagnosed when anthropogenic nutrient enrichment leads to an undesirable disturbance in the balance of organisms in the sea (e.g. changes in the species composition or abundance of phytoplankton). The effects of eutrophication propagate through the food chain and are often associated with decreases in water quality and fish health and abundance.

There are a number of European Directives and international conventions (e.g. OSPAR and HELCOM) that seek to eradicate eutrophication in Europe. EMECO partners include agencies who operate national marine monitoring programmes designed to provide the data for assessment of eutrophication. A formal assessment of eutrophication for OSPAR has recently been completed through the application of the so-called Comprehensive Procedure.

Monitoring Phytoplankton

Given their importance, it makes sense that scientists would want to closely track inter-annual and seasonal trends in phytoplankton numbers, in how they are distributed around Europe. Changes in phytoplankton composition or abundance can (but not always) reflect changes in nutrient levels.

In European marine waters, phytoplankton abundance, distribution, and species composition is monitored using a variety of platforms e.g. automated buoys, research vessels, the continuous plankton recorder (CPR), FerryBox systems, and satellite sensors. Observations are made on a number of temporal scales; research vessel survey may only sample an area a few days per year whereas automated buoys may be in operation for up to 80% of the year. Observations also vary in their spatial scales, for example, automated buoy make fixed-point observations, FerryBox systems tend to sample to same route repeatedly (e.g. Cuxhaven to Immingham), whilst satellite sensors monitor the oceans globally.

Chlorophyll a concentration is a widely used proxy for phytoplankton abundance. Chlorophyll a concentration is routinely measured in stiu, in the laboratory, and using remote sensing technology. Instruments that measure chlorophyll a concentration in situ detect either light absorbed, light reflected, or chlorophyll a fluorescence (e.g. FerryBox, SmartBuoy). To measure chlorophyll a concentrations in the lab, samples are collected on filter paper at sea. The Chlorophyll a is extracted from the filter paper using a solvent e.g. acetone. Chlorophyll a concentration is then measured using high-pressure liquid chromatography (HPLC), a spectrometer, or a fluorometer. In remote sensing radiometers mounted on spacecraft measure the optical properties of the oceans. Using algorithms, these optical properties can be used to estimate chlorophyll a concentration. There are a number of algorithms used to estimate chlorophyll a from remote sensing data, which are based on empirical observations (e.g. from HPLC) or validated optical models.