is the Primary production is the rate of synthesis of organic material from inorganic compounds such as CO 2 and water. It is significant in that it provides the base of most of the entire marine food chain. The formation of organic carbon compounds from inorganic carbon (e.g. carbon dioxide) involves a reduction reaction; the reducing power (e.g. NADPH) comes from either the absorption of light (photosynthesis), or the oxidation of other compounds (chemosynthesis). It is a rate, hence involves dimensions of time: mg C m -3 s -1, or in a depth integrated sense: mg C m -2 s -1

Open ocean produces like a desert on the land, but given its extension it accounts for the major global production. Estuaries and transitional waters are highly productive and each accounts for a 10% of the global net production

Note: PP is expressed as kilo calories per square meter per year

On a global scale open ocean production together with continental shelf largely exceeds all terrestrial productions

How is the rate of photosynthesis measured in the ocean? 1.Most common is the so-called 14 C technique. a.Collect water sample. b.Measure total alkalinity to detect the amount of total CO 2 c.Add radioactive inorganic carbon as a tracer as NaH 14 CO 2 d.Incubate under different light levels for some time (~ 1-24 hours) e.Filter sample, acidify to remove all inorganic carbon f.Measure radioactivity of what is left – this is proportional to the rate of fixation of carbon or primary productivity. g.Normalize to unit time and unit pigment concentration to express results (i.e. mol C (mg Chl) -1 h -1 ).

14 C Method: (Steeman-Nielsen, 1952) Labeled carbon is probably the most extensively used procedure for oceanic studies of productivity. This method essentially advantageous because it is relatively safe, weak β-emission (0.15 Mev) as well as its long half life (4700yr), so that storage offers no major problems. Procedure: The activity per ml of the working solution needed for the different productivity experiments depends on the production rates expected, duration of incubation, bottle size, etc. Invariably, ml of the working solution is used per bottle containing water sample.

Water samples for which production rates are to be determined are first collected from the specified depths and are transferred to the light and dark bottles kept in a dark box. Then, a known dose of the working solution is injected rapidly into the bottles with the help of a graduated hypodermic syringe having a needle not shorter than 5 cm, or better with an automatic dispenser. The bottles are then incubated for a known period by suspending them at the respective depths from where the water samples were taken for experimentation (in situ incubation). After the incubation is over, the experimental bottles are removed from the depths and are stored in a light-free case until the filtration of water samples is begun. Filtration may be done either on board the ship or in the laboratory. Aliquots of water samples for filtration are rapidly transferred into a suitable vacuum filtration apparatus on to a No. 2 membrane filter or Millipore filter of about 0.5 μm porosity. The vacuum should be applied at about 0.5 atm which will help avoiding damaging of fragile phytoplankton cells. The filtration should be done in a semi-darkend area.

The filters, after their removal from the filtration apparatus are placed onto planchets which are then kept in a desiccator containing silica gel. Filters obtained from light and dark bottles are then subjected to counting. To measure isotopes you need a liquid scintillation. After filtering and acidifying the samples the filters should be placed in scintillation vials and dried at room temperature for 24 hours.

Following the addition of scintillation liquid, the samples should kept in the dark for at least 3 hours to reduce chemioluminescence. The total carbon uptake is calculated from the equation: dpm(a) · total 12 CO 2 (c) · 12(d) · 1.05(e) · k1 · k2 dP(mgCL -1 hr -1 )= dtdpm (b) Where dpm(a) = Sample activity (minus back-ground), dpm(b) = Total activity added to the sample (minus back- ground), (c) = Total concentration of 12 CO 2 in the sample water, (d) = The atomic weight of carbon (e) correction for the effect of 14 C discrimination k1 = subsampling factor (e.g. sample 50 ml, subsample 10 ml: k1 = subsample factor 50/10=5) k2 = time factor (e.g. incubation time 125 minutes: k2= 60/125 = 0.48).

14 C technique measures something close to Net PP, particularly when incubation time is short. If incubation lasts too long part of the fixed 14 C can be respired and thus it is not more detectable by liquid scintillation. Accurately following the same method implies that data obtained in different region of the ocean can be compared. Warning: all operational steps must be done wearing gloves, using forcipes to handle filters. Filtration must be done under hood and the lab has to be periodically checked for isotopic contamination.

a.Collect water sample b.Measure initial oxygen concentration c.Incubate under different light levels (and dark) for some time. d.Measure final oxygen concentration e.Sum dark respiration to oxygen production in the light bottles to obtain Gross PP f.Normalize to unit time and unit pigment concentration to g.express results h.(i.e.mol O 2 (mg Chl) -1 h - 1 ). 2. Next most common is the measurement of oxygen evolution / uptake. It is far less sensitive than the 14 C method.

At the sea, before collecting water samples, you need to know the light profile, then you can choose standard depths or optical depths to collect water and after injection of 14 C (as NaH 14 CO 3 ) in the bottles to incubate them. It is better to select hours around noon to perform the incubation. On the basis of the daily solar radiation, obtained by means of surface sensor, you can extrapolate data obtained during the experiment to the daily primary production

In situ incubation Or in situ simulated incubation

The best “ in situ ” incubation is using a moored line from the bottom to the surface. This implies to keep your boat or ship stopped for the period of incubation. It is possible also to hang a line while the ship is moving. Otherwise you can use “ on the deck ” incubation using boxes with running sea water and screened with filters to mime light intensity of the depth of sampling. Problems arise when water temperature changes considerably with depth, as in well stratified situation. This can affect the photosynthetic processes.

Simulated “In situ” incubation Photosynthetron: Controlled laboratory incubation (Lewis and Smith 1983) ICES incubator At the land laboratory you can use different devices at fixed temperatures. You can regulate light intensity to again mime light intensities registered along the water column.

P vs E curves experimentally obtained with water from the Gulf of Trieste with an ICES incubator. Note that in May light intensity produced in the incubator was enough to reach photosaturation, so that it was possible to apply Eilers-Peeters function, while in June there was no saturation and thus the linear Platt-Jassby function was applied.

Comparison between in situ and with incubator PP production. Often at the surface the two rates do not properly fit.

Temporal patterns of integrated PP measured in situ at two coastal adriatic sites (Gulf of Trieste) in the period January December Note the inter-annual differences (in 2000 PP tripled) and among the the two stations (AA1 closer to river inputs is more productive). There are usually two seasonal maxima: From Fonda Umani et al., AME 2007 the first in late winter - beginning of spring and the second in fall.

PP at different depth measured in the Gulf of Trieste with in situ incubation following the 14 C method (left). PP climatology ( )