MARS 4200 Exam 3: Subtropical Gyres

What taxonomic groups of phytoplankton typically bloom in subtropical gyres? Who are the dominant herbivores? How important is the microbial loop here?

-Cyanobacteria (Pro, Syn), diatoms, dinoflagellates -Herbivores Ciliates, Copepods, Salps ML -Is very important due to the importance of recycling -Especially the cycling of dissolved organic matter (DOM) back to regenerated nutrients

How do seabirds, fish, marine reptiles, and marine mammals depend on the subtropical gyres?

-Dominated by long-lived species that are constantly grazing -Seabirds that are constantly flying are selected for because there is no land -The waters are clear, so organisms in this area tend to feed once a day during the night when its safer. Diurnal organisms

Explain the concept of the f-ratio and why this ratio is so low in subtropical gyres

-F-ratio = new primary production from new nitrogen (NO3-)/ Total primary production (all N sources)

Geostrophic flow

-Inward deflection by Coriolis is balanced by gravity (outward/downward pressure) -Coriolis (inward/upward) = Gravity (outward/downward) -Circular motion

Draw a typical chlorophyll a profile for the upper 200 m of a subtropical gyre and compare it to a typical productivity profile. Is there a difference? Why?

-Low-light adapted cells at the base of the mixed layer have a higher amount of chlorophyll in them. This is where the maximum chlorophyll a is at. For production, the maximum (most carbon fixation) is at the surface. -The amount of chlorophyll or biomass that is there is not the same as how much productivity is happening

What is the role of nitrogen fixation in subtropical gyres? Compare the North Pacific to the North Atlantic subtropical gyre.

-Nitrogen fixation is a biological process in which certain microorganisms, known as diazotrophs, convert atmospheric nitrogen gas (N₂) into biologically available forms like ammonia (NH₃) or nitrate (NO₃⁻). In subtropical gyres, which are characterized by oligotrophic (nutrient-poor) conditions, nitrogen fixation plays a crucial role in supplying new nitrogen to the marine ecosystem. -More nitrogen fixation happens in the Atlantic because there are more diazotrophs due to the amount of iron being higher (dust from Africa) and the amount of nitrate being lower

Recycling of nutrients

-Since nutrients are low, organisms have gotten good at using them efficiently -Production can be supported by new (nitrate, No3-sourced) or regenerated (ammonia, NH4+) -Very little production in the gyres by new N (NO3) -Ammonium (NH4+) is an important source of nitrogen in subtropical gyres

What physical ocean features characterize subtropical gyres?

-Subtropical gyres have water column stability (barotropic conditions, low vertical mixing, permanent thermoclines) -This means it is "oligotrophic"/ low nutrients/ low biomass at the surface. -Gyres exist due to the physics of wind forcing. Winds deflected to the right in the northern hemisphere and to the left in the southern hemisphere

Discuss the length of the food chain/web in the subtropical gyres compared to say, upwelling zones. How does this affect the size of the fishery ? Would you expect a higher or lower sustainable yield from subtropical gyres?

-The subtropical gyre is considered an open ocean. Open oceans tend to have more steps within its food change. This means that by the time the biomass reaches the highest trophic level, there is very little left. Upwelling areas however have smaller food chains, which means the higher trophic levels get more biomass once it reaches them. -I would expect lower sustainable yield from subtropical gyres because coastal areas are usually better for fisheries. -Coastal areas support a higher sustainable yield and bigger and economically larger fishery

Barotropic conditions

-Water column stability -Water gets piled into gyre center where it is warmed by the sun -Vertically "flat" density surfaces vs depth over large distances -Low-density (warm) waters at the surface and higher-density (cooler) waters at depth -Gravity and coriolis are balanced. No gravity-driven mixing. Ocean is "stratified".

Permanent thermocline

-Water column stability (little vertical mixing) -Strong stratification contributes to the persistence of surface waters in subtropical gyres, allowing them to remain warm and nutrient-poor -Nearly uniform salinity too (weak halocline)

Ekman Transport

-Wind blows across water -The surface currents created by the winds induce a phenomenon known as Ekman transport, where the surface water is displaced at an angle (90) to the direction of the wind due to the Coriolis effect. In the Northern Hemisphere, this transport results in a net movement of water to the right of the wind direction, while in the Southern Hemisphere, it is to the left.

What is the Pacific Garbage Patch? What harm does it do? What are some solutions?

-is a collection of marine debris. also refers to the garbage patch as a vortex of plastic waste and debris broken down into small particles in the ocean and becomes trapped. -can be ingested by marine animals or become entangled in it -microplastics block sunlight from reaching plankton and algae below. -If algae and plankton communities are threatened, the entire food web may change. -limiting or eliminating our use of disposable plastics and increasing our use of biodegradable resources will be the best way to clean up

What is a "deep chlorophyll maximum (DCM)?" Why is it regularly observed in subtropical gyres?

DCM increases until it hits the mixed layer line and then you get a deep maximum right under the mixed layer, then it declines until it reaches 0. There is a lot of Chl at the base of the mixed layer because light levels are low and some nutrient availability. DCM is a maximum in pigments, not necessarily of primary productivity.

How does the physical regime determine the nutrient availability in the subtropical gyres? Be able to draw a typical nutrient profile alongside a temperature profile (and label both axes!).

Nutricline: Stratification leads to a strong and persistent drawdown of nutrients at the surface and a consistent increase in nutrients with great depth. Nitrate and phosphate low at surface and increases with depth below the mixed layer. Nutrients at the surface are low because primary producers are taking it up and building new biomass

What is the seasonal cycle of productivity at the HOTS station? How does it differ from seasonality at BATS? Why?

There is not much seasonality at the HOTS station because there is very little to no winter mixing and there is a slight bump due to summer warming and shallow MLD. At BATS, there is more seasonality due to modest winter mixing from cooling and stroms.