Marine production and food security

Food Security

Food security is dependent on stable fish and shellfish populations. These populations are affected by bottom-up and top-down control as they are part of ecosystems with trophic structures where energy is produced at the lower levels (primary production) and consumed at the higher levels. Food security is in fact dependent on maintaining stable ecosystems.

Planktonic algae

The planktonic algae in the Oceans are microscopic, eukaryotic, unicellular organisms ranging in size from 0.2 µm to 2 mm (2000 µm), a range comparable to that between the smallest fish and the biggest whales. They are dependent on trace elements, nutrients, light and CO₂ to perform photosynthesis, a process that produces organic carbon, oxygen as a waste product, and that forms the basis for almost all higher trophic organisms in the oceans. The rate of oxygen or organic carbon production is called primary productivity.

      CO₂ + H₂O + Light                               CH₂O + O₂

Biomass in algae is often measured as Chlorophyll a concentration. Chlorophyll a is the main pigment for photosynthesis and all photosynthetic algae contain it. Chlorophyll a biomass can be calculated by measuring how much light a sample absorbs. The pigments in the sample are first extracted in an organic solvent and are also often separated (from other pigments) by chromatography to get a clean reading of the Chl a concentration.

Another way to measure the Chl a concentration is by measuring the fluorescence from the extract (or from the live sample - but this is less accurate). Fluorescence is a property of Chl a just as the absorbance. When the energy from the light cannot go to photosynthesis it is released as fluorescence or heat. This creates a red light that can be measured by a light sensor. In an extract the fluorescence signal is strong, but it can also be detected on a live sample.

The fluorescing properties of Chl a have many applications and is very often used in situ for detecting algae biomass from platforms such as buoys, FerryBox, profiling CTDs or autonomous underwater robots (AUVs, gliders).

The spring bloom in Norwegian Coastal Waters

The spring bloom is generally the first bloom after the winter season, utilizing the nutrients stored during winter when light is generally too low to support a bloom. Even though light and availability of nutrients are essential factors, stability of the water column is important for the bloom to start. Therefore, the start date for the spring bloom is difficult to predict. The Norwegian Coastal Current is flowing like a river along the coast from south to north, and in the fjords we have the estuarine circulation, so the water at a location is not stationary. Therefore, incidents of no registered bloom despite exhaustion of nutrients at a location may occur.

Generally, the spring bloom starts earlier in the south than in the north. In the Skagerrak area small winter blooms occasionally occur in January, but the start of an early spring bloom is generally at the beginning of February and on the west coast a few weeks later. A late spring bloom may occur in April or even May. In the north an early spring bloom may start in mid-March or beginning of April. The time span for the spring bloom to take place is wider in the south than in the north. On average the start of the spring bloom in the southern part of Norway is during the period from mid-March to late March, and in middle and northern Norway from end of March to mid-April. Due to the difference in time span, occasionally the spring bloom in some areas in the south may start later than in the north.

Cell-carbon – another algal biomass parameter

Planktonic algal biomass can be measured in several ways. Chlorophyll a is the most used biomass parameter because it is easy to measure and cheap. However, chlorophyll a is just a proxy for the algal biomass, and the content of chlorophyll a in algal cells varies considerably with light conditions, nutrient supply, the algal composition etc. Another more accurate parameter expressing the number of algae in the water column is the algal cell-carbon. Cell-carbon can be calculated based on microscopic analyses of water samples where: i) the different phytoplankton species are identified and quantified as cell numbers per unit volume, ii) cell numbers are converted to biovolume using best fitting geometric shapes and matching equations for each species, iii) biovolumes are converted to cell-carbon using accepted empirical formulae (e.g. Menden-Deuer & Lessard 2000).

Calculated cell-carbon is not affected by light, nutrient supply or any other environmental factors. In biological modeling, carbon is the key unit. Cell-carbon is expressed as carbon per unit volume and avoids introducing sources of error via assumed carbon: chlorophyll ratios. In addition, cell-carbon gives information about which species/algal class/functional group is the most important when it comes to biomass contribution, while chlorophyll a only informs about the total biomass.