Chapter 2: primary production processes

2.9 global trends in primary production

• four major factor that govern primary production in marine systems:

  • light → only available in the upper part of the water column (photosynthesis till 200 m)

  • nutrients → that can be exhausted in the upper water layers, but are generally available int he deeper part of the water column

  • stability → to allow algal growth in the surface water layers

  • mixing → to replenish used nutrients from lower water layers to surface

• at the end, it is the physics of the water body that control primary production

• there are many oceanic processes that influence nutrient supply to surface water:

  • storms that mix stratified waters

  • global thermohaline circulation patterns (the global oceanic conveyor belt)

  • cyclic ocean events like el nino and la nina

• irradiance → the amount of light energy available/ able to pass through into the water for organisms to photosynthesise

• in tropical and subtropical waters:there is higher thermal stratification due to the higher amount of solar heating

  • there is higher thermal stratification due to the higher amount of solar heating

  • in clear waters there is high irradiance, as the light energy is able to reach depths greater than 200 m sometimes

  • however, due to stratification beign so significant and high in these water, teh amount of nutrients in the upper layers of tropical waters is less

  • this makes tropical waters areas of low primary production zones

  • in turn, when there are storms, tropical waters are more susceptible to eutrophication, since the high light energy and nutrients provide a great environment for primary producers to thrive (aka, algae)

• polar oceans:

  • primary production is restricted to short seasonal windows when light is available

  • there are differences in the nutrient status between the arctic ocean and the southern ocean.

    • in the southern ocean, nitrogen and phosphate are in excess, but primary production is restricted due to a limitation of iron

    • in the arctic, nutrient limitation occurs annually in the late spring/summer plankton blooms

• temperate water:

  • varies seasonally

  • spring and autumn phytoplankton blooms, with low amounts of phytoplankton in the summer and winter.

• anticyclonic gyres →

  • clockwise in the northern hemisphere and anticlockwise in the southern hemisphere

  • they deepen the thermocline, moving surface water down to the centre of the gyre, therefore nutrient replenishing in the surface does not take place

  • these gyres are regions of low primary production

• cyclonic gyres →

  • anticlockwise in the northern hemisphere, clockwise in the southern hemisphere

  • water is transported from the bottom to the surface, from the centre of teh gyre and outwards

  • higher rates of primary production is supported, due to the mixing of nutrient rich waters into the surface

• what happens in cyclonic gyres is called upwelling and what happens in anticyclonic gyres is called downwelling

• regions of upwelling are important sites for primary production

• coastal waters and waters overlying the continental shelves support the greatest primary productivity

  • this is because these areas are shallow, so it doesnt take much for the sunlight to penetrate all the way down to the bottom in these areas. so basically the entire water column gets light for primary production

  • also nutrients are brought in through the surrounding land mass, where there is always some sort of nutrient runoff into the water, either in small amounts or large amounts during storms. there are rivers that lead into the sea, bringing the nutrients with them.

• frontal systems → they are regions of enhaced biological activity, either from increased primary production or a concnetration of organisms through physical processes

  • this is where different water masses meet.

  • so if oceanic gyres are vertical movement of nutrients, frontal systems are horizontal movements of nutrients

  • these areas are known to be hotspots of primary production, and in turn hotspots for animals to feed. For example:

    • when phytoplankton aggregate in this area, we get zooplankton that feed on phytoplankton and plankton eating fish. These fish are predated on by whales and birds that come into these places to feed on said fish.

2.9.1 global ocean primary productivity

• global ocean net primary productivity estimates are numerous and varied

• the variability is due to the methods used to measure ocean primary production and the different metabolic processes that these methods quantify

• marine and terrestrial primary production are roughly the same, at approximately 50 Pg C y^-1 

• macro algae can produce up to 14 kg carbon m^-2 y^-1 

• with modern day satellites, it is possible to estimate the chlorophyll concentrations in the surface waters of the world’s oceans, and therefore monitor phytoplankton growth/blooms

box 2.11 comparison of terrestrial and aquatic primary production

• although phytoplankton productivity is much less than that of macro algae per unit are, on a global scale total phytoplankton productivity is far greater than that contributed by macro algae

• terrestrial systems have a higher global annual primary production compared to marine systems

2.9.2 are the oceans net heterotrophic or net autotrophic?

try to go over this section again by putting this sub section into ChatGPT

• as there is no photosynthesis below 200 meters depth, metabolism needs to be supported by falling organic material out of the euphotic zone

• may expect a general decrease in activity with time and depth

• the epipelagic zone contributes the most when it comes to respiration, according to table 2.5

• very little of the carbon assimilated in the euphotic zone through primary production actually gets sequestered in the sediments, since most of it is respired as it sinks

• zooplankton are a vector for the transport of organic material

• the mass-balance calculation leads us to the conclusion that there is a net input of about 20 Tmol carbon per year to the oceans

• thus the oceans are most probably net heterotrophic with respiration exceeding photosynthesis by about 0.2%

2.10 primary production in seaweeds

• seaweeds on shore can grow at very fast rates, amassing large biomasses

• the productivity of seaweeds and seagrasses is equal to or in some cases greater than terrestrial plant systems

• in water, seaweeds obtain their carbon dioxide through CO2 or from HCO3^- 

• when exposed to air, their only source of carbon is carbon dioxide from the air

  • as long as the seaweeds don’t dry out, they will be able to photosynthesise at similar rates as to when they are submerged underwater

  • however, when they begin to dry out, at least for some species, photosynthesising rates are reduced significantly.

  • there are some species that are able to withstand desiccation even though water is lost around them by nearly 35%.

    • this is common in species in intertidal zones, like the Fucus species

• when submerged, considerable energy is expended in maximizing the amount of light getting into the chloroplasts

• there are several morphological features to counteract this problem:

  • stipes

  • pneumatocyst

• stipes →

  • they are kind of like trunks for the kelp, which supports most of the photosynthetic tissue at the surface

• pneumatocyst 

  • they are gas filled air bladders that ensures that the blades are floating as high as possible in the water

  • the gases within the bladder contain oxygen and nitrogen in roughly the same proportion as in the air, and varying amounts of carbon dioxide

• the stipes of the seaweeds act as surfaces for epiphytes to grow on

  • it can vary from macroalgal fonds through to biofilms, and macroalgal assemblages, colonial animals like bryozoans

• epiphytic growth can reach to such densities that it can prevent the exchange of gases and inorganic nutrients, and cut down on light that can reach the photosynthetic cells

  • it can also increase drag to an extent that the seaweed/seagrass is washed away with the strong currents

• to avoid these problems, the seaweeds perform this mechanism called “skin shedding” → layers of cell walls are shed from the outer surfaces of the seaweed carrying the epiphytes

• seagrass blades are also known or supporting epiphytic growth.

  • they deal with this by producing a larger number of shoots, and the blades that have the epiphytes are broken off and lost.

revision questions

section 2.9

1. high irradiance means that the light can penetrate all teh way past 200 m. and in tropical regions, they are well known for their high temperatures. the high irradiance heats up the surface of the water column, creating stratified water masses, making it difficult for nutrients to be mixed around between these water masses, therfore making it a low productivity area.

2. teh amount of light, the amount of nutrients that get washed off form terrestrial land and into the water, due to the seasonality, like rainy seasons, or sumemr seaosns which are dy and doesnt really have much rain to help with nutrient run off

3. polar regions are well knwon to have long periods of darkness and long periods of sunlight. during the sumemr time, the amount of sunlight thats available is mostly the entire day to 24/7. this allows teh phytoplankton to take opportunity of the light availability and photosynthesize.

section 2.10

4. the amount of light that is available in the given area. seaweed depend on sunlight to photosynthesize completely. 

5. turbulence helps move the baldes of the seaweed, allowing differents part of the seaweed to reach the light, giving exporusre to different areas on the balde. turbulent waters are bring in nutrients, inorganic minerals, and carbon compounds and taking awya the repsired compunds to and from teh seaweed.