Overview

Iron and its supply

- Before the evolution of photosynthesis, Iron (Fe) was highly abundant in the world's oceans. Because of oxygenic photosynthesis, the oxidation of Fe occurred, and in this form Fe is virtually insoluble in oceans. This is a critical reason why despite its high abundancy in the Earth's crust, the bioavailability of Fe is dependent on its redox state. - Reduced form is soluble, oxidized form is not. - Major source of iron is wind-blown from terrestrially derived dust. - Over the past several years, it has become increasingly apparent that, for the ocean as a whole, N2 fixation is itself limited by some factor, probably Fe. Therefore, marine N2 fixing species are sensitive to Fe levels. - The transport of aeolian Fe is related to wind speed and direction, which are related to the temperature contrast between the continents and ocean Silica supply, essential for diatom blooms, is primarily dependent on riverine fluxes and upwelling from the ocean interior. The distribution of N -fixing cyanobacteria is not only dependent on Fe fluxes but also a warm, relatively quiescent euphotic zone. Food-web structure, which is a strong determinant of export flux, is critically dependent on mesoscale physical processes that promote nutrient pulses. (Falkowski et al., 1998)

Ocean Nutrient Inputs

- atmospheric transport - rivers (evident by river plumes) - nitrogen fixation - Iron inputs into the ocean, particularly in the Sahara desert

Factors that influence the water cycle

- weathering - biological activity (plants and animals) - ocean biological activity - human activity (industry, agriculture/agrochemicals)

How will climate change affect primary productivity?

--> likely to affect by altering nutrient availability and light. --> The effects may include shifts in the timing of the start, end or peak of the growing season, in addition to changes in the amplitude of the seasonal cycle (Henson et al., 2013). --> are likely to have knock-on effects for higher trophic levels via the "match-mismatch" hypothesis (Cushing, 1990), as late blooms may result in a reduced period of time when prey (phytoplankton) are available to predators (zooplankton or larval fish), and vice versa for earlier blooms. In the northwest Atlantic, for example, the survival of haddock larvae is closely linked to the timing of phytoplankton bloom initiation (Platt et al., 2003), and it has also been suggested that the annual shrimp hatching has evolved to coincide with the mean bloom start date (Koeller et al., 2009). Current patterns of phytoplankton phenology are set partly by the population's response to nutrient and light availability. At high latitudes, deep winter mixing results in light limitation of phytoplankton growth but at the same time ensures a plentiful supply of nutrients, so that shoaling of the mixed layer in spring results in rapid, sustained growth, i.e. a strong seasonal cycle. This is in contrast to subtropical regions where mixed layers are sufficiently shallow year-round such that light limitation does not occur. However, this lack of mixing results in nutrient limitation through much of the spring and summer, and phytoplankton blooms are only stimulated when winter mixing or storms deepen the mixed layer sufficiently to entrain new nutrients, resulting in a weak seasonal cycle. In this context, continued global warming, leading to increased stratification, is hypothesised to reduce the seasonal magnitude of ocean primary production in nutrient-limited regions

Seawater nutrient composition

--> macronutrients have special vertical profiles in different bodies of water (Andrews et al.) --> No3 (nitrate) at surface is often extremely low due to large population of algae there. --> some organisms have a greater range of adaptability to certain nutrients. For example, cobalt has a much greater range when it comes to cellular ratio, however with nutrients like N, only small error bars exist because it is a limiting nutrient (Moore et al., 2013).

3 main types of phytoplankton

1. Diatoms (relatively big and have a SiO2 skeleton_ 2. Coccolithophores (CaCO3 skeleton and produce massive blooms that can be seen from space) 3. Dinoflaggellates (photosynthetic with an organic skeleton). These form a euphotic zone food web.

Factors that allow plankton growth

1. Light 2. Water 3. Inorganic carbon (not a limiting factor due to its abundance) 4. Nutrients (N, P, Silicon, Iron, etc.)

Factors that influence dissolved oxygen in surface waters

1. Solubility = equilibrium with the atmosphere 2. Organic carbon pump = the ocean's biologically driven sequestration of carbon from the atmosphere to the deep ocean. It is the part of the oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis. 3. Time since water mass at surface, influenced by the thermohaline circulation of water.

Effect of iron on primary producvity

Another factor recently discovered to have an effect on primary productivity is Iron. This micronutrient is used as a cofactor in enzymes involved in processes such as nitrate reduction and nitrogen fixation. A major source of iron is blown into oceans from deserts, particularly the Sahara. In regions that are not near deserts such as the Southern and North Pacific oceans, iron can be a limiting factor in primary production.

Redfield Ratio

Defines relative rates of uptake and regeneration of critical elements in ocean biogeochemistry and their close coupling. 106:16:1 C:N:P To sustain the C flux through marine ecosystems, essential nutrients must be supplied. The mean elemental ratio of marine organic particles is 106C/16N/1P by atoms and is highly conserved, known as the Redfield Ratio. Such a specific elemental ratio in organic matter is unique to ocean ecosystems and reflects the average biochemical composition of marine phytoplankton and their early degradation products.

Ocean production

In the ocean, almost all productivity is performed by algae, with a small fraction performed by vascular plants and other groups. Algae encompasses a vast array of diverse organisms which can range from single-celled algae to seaweed. Majority of photosynthesis is performed by phytoplankton. The factors limiting primary production are availability of water, temperature, availability of light, and nutrients.

How do nutrients affect primary productivity?

Inorganic nutrients such as nitrate, phosphate, and silicic acid are necessary for phytoplankton to synthesise their cells and carry out cell processes. Because of the gravitational sinking of particulate material such as dead organisms and fecal material, nutrients are constantly lost from the photic zone and are only replenished through the upwelling of deeper water. In summer months, stratification of the water column increases due to more solar heating and reduced wind. This strong thermocline can make it difficult to have mixing occur, as a consequence, the available nutrients will be consumed and may lead to reduced productivity.

Sequence of reactions as oxygen is depleted

OXIC: = contains 250-300 mmol of oxygen Here, aerobic respiration occurs so oxygen is consumed and CO2 is produced. Oxygen is also consumed to produce nitrous oxide. HYPOXIC: = < 60 mmol of oxygen and metazoans start being oxygen stressed SUBOXIC: = <5-20 mmol of oxygen Here, dentrification occurs where nitrate is consumed, and nitrogen is produced via nitrous oxide. Fe3+ is reduced to Fe2+ and ammonium is produced. These zones are known as oxygen minimum zones, which are expanding! ANOXIC: = no detectable oxygen. Here, sulphate reduction occurs and methane is produced, along with hydrogen sulphide.

Marine Phosphorus Cycle

Phosphorus is almost totally absent from the atmosphere, and the only significant input of phosphorus to the oceans comes via river water. The most significant output of phosphorus from the oceans is in organic debris sinking to the ocean floor and becoming incorporated into sedimentary rocks. The most important processes responsible for the internal distribution of phosphate in the oceans, are as follows: (1) the assimilation of dissolved phosphate into phytoplankton biomass; (2) the subsequent release (through grazing, cell lysis, bacterial degradation, and so on) of most of this phosphorus back into dissolved nutrients in the surface ocean; (3) the release of a further portion of the organic phosphorus back into deeper waters after organic material has sunk out of the surface oceans; and (4) slow mixing between the surface and deep oceans, partly due to the physical movement of large masses of water between the surface and deep layers (upwelling and downwelling), and partly due to diffusion. (Tyrell, 1999)

Effect of light on primary productivity

The sunlit zone of the ocean (euphotic or photic zone) is a relatively thin layer that extends up to 100 m. The most influencing factor on the amount of light is the seasonal cycle. The tropical regions close to the equator do not have a large variation in sunlight throughout the year. Conversely, primary production in temperate regions such as the North Atlantic is very seasonal, varying with the amount of light at the water's surface (less in winter) and ocean mixing (higher in winter).

Marine Nitrogen Cycle

This is more complex than the phosphorus cycle. Nitrogen exists in several dissolved forms in the ocean, including - nitrate (NO3−, typically present at 0-40 µmol N kg−1) - nitrite (NO2−, 0-1 µmol N kg−1) - ammonium (NH4+, 0-1 µmol N kg−1) - dinitrogen (N2, ∼1,000 µmol N kg−1) - dissolved organic nitrogen (DON) compounds not directly accessible by phytoplankton. 'Reactive nitrogen' is the only form of nitrogen explicitly modelled here, and refers to the sum of the forms of nitrogen that are easily taken up by phytoplankton: NO3−, NO2−and NH4+. N2 concentration (requiring greater energy expenditure to access, due to the strong triple bond in N2) is not explicitly included in the model because it is always present in excess in the water. Another difference from the marine phosphorus cycle is the significant loss of reactive nitrogen due to denitrification, which transfers reactive nitrogen back to N2. For simplicity, other N and P processes of lesser importance are omitted from the model, although considered in sensitivity analyses. (Tyrell, 1999)

Coastal Sea

around 11% of the total ocean but accounts for 29% of total productivity (Longhurst et al., 1995).

How does temp and salinity influence solubility of gases?

lower temp = more solubility more salinity = lower solubility

Nitrogen fixation

main way nitrogen enters the ocean. accounts for 70% of the atmosphere and is due to specialised bacteria. the breaking of chemical bonds requires light, Fe, and P.