Chapter 11 Key Concepts

In midlatitudes, phytoplankton increase in the winter and spring, decline in summer, and may increase to a lesser extent in fall.
Zooplankton start to increase as the phytoplankton bloom reaches its peak, attaining a maximum following the phytoplankton peak in the late spring or early summer.
The spring phytoplankton peak and the later zooplankton peak are shortest and sharpest in high latitudes, becoming indistinct in the tropics.
Because light intensity decreases exponentially with increasing depth, there is a compensation depth below which respiration for a given phytoplankton cell exceeds photosynthetic output.
During winter, the water density is about the same throughout the water column, and phytoplankton cells are stirred on average to depths that prevent average photosynthesis from permitting the phytoplankton population to become dense.
As the spring temperature increases, the surface waters warm up, and the water column stabilizes and allows the phytoplankton bloom to take off. But eventually, nutrients are lost to deeper waters, and the phytoplankton bloom is cut off.
The mixing depth–critical depth hypothesis has been criticized because phytoplankton blooms often develop in late winter, when waters are still strongly vertically mixed.
In very-shallow-water estuaries, nutrient exchange, or benthic-pelagic coupling, occurs between the bottom and the water column, fueling more phytoplankton growth.
In estuaries, the spring freshet combines with net water flow to the sea and water mixing to determine the nutrient regime.
Light may be inhibitory to photosynthesis near the surface, but a series of photosynthetic pigments captures light over much of the visible spectrum.
Photosynthesis increases with increasing light intensity, up to a plateau, and then is inhibited by high light intensity.
Nutrients are required by photosynthetic phytoplankton. They may occur in dissolved and particulate form.
Nitrogen is required for protein synthesis and is taken up in the forms of ammonium, nitrate, and nitrite.
Nitrogen supplied to phytoplankton can be divided between that provided from new production and that provided from regenerated production.
Nitrogen recycles between phytoplankton and the bottom in shallow-water environments. Zooplankton excretion is another major source of recycling.
Nitrogen cycling is intimately involved with microbial transformations.
Phosphorus occurs in seawater mainly as inorganic phosphate, is required for the synthesis of ATP, and is a crucial energy source in enzymatic reactions.
The nitrogen-to-phosphorus ratio in the sea is generally about 14.7:1 and is regulated by uptake and decomposition of phytoplankton, whose N:P ratio is about 16:1.
Iron is a crucial limiting nutrient element needed in the synthesis of cytochromes, ferredoxins, and Fe-S proteins.
Iron normally is insoluble in typical ocean water and must be kept in a chemically complexed form to be taken up by phytoplankton.
Iron is the important limiting nutrient in high-nitrogen, low-productivity areas of the ocean, which are remote from terrestrial windborne sources of iron.
Experiments at sea with iron addition demonstrate rapid mesoscale increases of primary production.
Nutrient uptake varies with taxonomic group of phytoplankton, cell size, and conditions of microturbulence.
Nutrient uptake increases with increasing nutrient concentration, eventually leveling off to a plateau.
Inshore phytoplankton live at higher nutrient concentrations and would be expected to be able to take up nutrients at higher nutrient concentrations in the environment relative to open-ocean phytoplankton, which would be expected to be more efficient at low concentrations.
A stable water column, input of nutrients, and sometimes an initial input of resting stages all combine to promote dense, harmful phytoplankton blooms.
During the production season, there is a successional sequence of phytoplankton species whose general properties correspond to the seasonal trend of nutrient availability. Differential dependence on substances excreted by phytoplankton and production of toxic substances may influence the succession of phytoplankton species during the production season.
Coexistence of many photosynthetic and heterotrophic microbial groups under nutrient limitation presents an ecological paradox of coexistence.
The roles of nutrient concentration and turbulence can be integrated into a general model of phytoplankton dominance.
Phytoplankton may take up organic molecules, but bacteria are the major heterotrophic consumers in the water column.
Protists are the major consumers of water column bacteria and are themselves a major component of the food of zooplankton.
Zooplankton abundance usually follows phytoplankton abundance in coastal and shelf planktonic systems. Although zooplankton growth depends on phytoplankton growth, zooplankton grazing can, but often does not, control phytoplankton abundance.
Feeding behavior varies with phytoplankton cell size and cell concentration (cells per unit volume).
Zooplankton can select particles by size. Owing to the low Reynolds number for copepods, this involves direct plucking of preferred particles rather than straining of particles on a feeding sieve.