Lect12_ZooMicroLoopTwilight

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Sediment traps are used to measure sinking fluxes in the oceans. Cones with the open face upward catch sinking material and collect in cups at base of cone. Often the cups rotate out on monthly timescales for deeper traps. Trap can be deployed for ~ a year and collect monthly data on the sinking flux. Preservatives in cup keep organic material from remineralizing. Problems with sediment traps include size selectivity due to current shear, and "swimmers".

(from Berelson, 2001)

The Mineral Ballast Hypothesis of Armstrong et al. (2002) suggests that the flux of POC to the deep ocean is a function of the production of POC in surface waters AND of the amount of mineral ballast present in the sinking material. The three types of mineral ballast are dust particles from atmospheric deposition, biogenic Silica (bSi) from the diatoms and radiolarians, and calcium carbonate (CaCO3) from coccolithophores, foraminifera, and pteropods. The mineral ballast suggests that community composition may be an important factor in POC flux to the deep ocean and to ocean sediments. Alternate view suggests that it is POC (and TEP) production that allows small ballast fragments to aggregate and sink to the deep ocean (i.e., De La Rocha and Passow, 2007).

(from Armstrong et al., 2002)

Macrozooplankton: Some key groupsAll the macrozooplankton feed on larger phytoplankton, microzooplankton, and larger detrital particles. Copepods: most common macrograzer, type of crustacean, range in size from a few mm to several cm, important players in the biological pump: 1) formation of large fast sinking fecal pellets that export organic matter to deep ocean, and 2) some species vertically migrate over diel cycles, typically feeding near surface at night and moving to deeper waters during the day. Calanoid copepods often comprise more than 70% of the macrozooplankton biomass caught by nets.

The copepods have 12 juvenile stages in their life cycle, six nauplii and six copepodite stages. The progression from egg to adult typically takes several months to a year, varies by species.

Macrozooplankton: (cont.) Euphausiids: another group of crustacean, small shrimp-like grazers, includes Antarctic Krill important grazer in Southern Ocean. Foraminifera (Forams) amoebae that have a calcareous (CaCO3) shell (or test), key player in the carbonate pump, shells accumulate in sediments and are often used in paleoceanographic studies (i.e. sea surface and subsurface temperatures). Pteropods planktonic snail with a calcareous (CaCO3) shell, a secondary player in the carbonate pump, less abundant than Forams, but shells do accumulate in some ocean sediments.

Macrozooplankton: (cont.) Radiolarians spherical, amoeboid protozoans with a skeleton made of biogenic Si, secondary player in the ocean Si cycle after the diatoms (a favorite food). Silica components accumulate in ocean sediments. Salps: gelatinous filter feeders, that can form large swarms, and also make fast-sinking fecal-pellets. Larvae: fish larvae and larval forms of many benthic species like clams, snails, barnacles, etc. spend the early part of their lives as macrozooplankton.

Krill are particularly important in Southern Ocean waters where they form massive swarms that can clear the water of phytoplankton. They are an important food source for all higher trophic levels in this region including fish, birds, whale, seals, etc... Mature krill winter at depths of hundreds of meters, juveniles spend the winter beneath the sea ice.

(Photo Larry Madin, WHOI) (Feely et al., 2004) Pteropods (pelagic snails) also referred to as Sea Butterflies, or Sea Angels, form shells of CaCO3, and may be threatened by ocean acidification. They an important food source for a diverse group ranging from larger zooplankton, to salmon, to baleen whales.

Foraminifera (aka forams) are single-celled protists that form CaCO3 shells. Foraminifera may be threatened by ocean acidification. Their shells are often preserved in the sediments and are useful for paleo-oceanography. Some species live only within a relatively narrow temperature range. Paleotemperature can be reconstructed based on species present in sediments.

Microzooplankton: Some key groupsHeterotrophic Dinoflagellates large dinoflagellates adapted for particle ingestion (grazing). Some are mixotrophs capable of photosynthesis and grazing. Zooflagellates diverse group of smaller flagellated protists, feed on pico-plankton, bacteria, and detritus. Ciliates common grazers, covered with cilia for locomotion, large species part of macrozooplankton, feed on smaller microzooplankton, phytoplankton, and bacteria.

Marine Heterotrophic BacteriaStill a lot we dont know. Consume significant % of primary production, generally less important in cold, polar waters. Historically bacteria were enumerated with microscopes. In late 1970s-1980s we learned that many of these bacteria: were actually autotrophic plants, had chl. A fluorescence, were metabolically inactive, either dormant or dead. DAPI and AO (acridine orange) stain DNA. A variable fraction of bacteria dont have active DNA, but some of these are dormant and can recover. Not well understood what converts dormant to active, possibly related to nutrient stress.

Dissolved Organic Matter (DOM): Dissolved organic material released to the marine environment mainly from phytoplankton through excretion, and as a byproduct of grazing or viral lysis. Dissolved Organic Carbon (DOC): carbon portion of Dissolved Organic Matter. DOM is remineralized by heterotrophic bacteria into inorganic carbon and nutrients. There are three classes of DOM: labile easily broken down on a timescale of hours to days. semi-labile broken down by bacteria on a timescale of months. refractory tough for bacteria to breakdown, lifetime of hundreds to thousands of years, significant carbon pool in the deep ocean. Under strong nitrogen limitation, phytoplankton excrete DOM with high C/N ratios (>> Redfield values, 10-20)

The Microbial Loop refers to the cycling (and recycling) of elements between inorganic nutrient pools, phytoplankton, dissolved organic pools, bacteria, and the microzooplankton. The mid-ocean gyres where stratification is strong, nutrient inputs are low, and the phytoplankton community is dominated by picoplankton have a very active microbial loop. Some material flows through the microbial loop in all marine systems. The trophic transfer efficiency from phytoplankton to zooplankton is ~25-30%, at higher trophic levels this decreases to ~10%.

Phytoplankton (large)

(all)

DOM

Heterotrophic Bacteria

Macrozooplankton

Microzooplankton

Flagellates

Small Fish

The Microbial Loop (Pomeroy, 1974; Azam et al., 1983)

Larger Fish, Higher Trophic Levels

Classic Ocean Food Chain

Azam et al. (1983) Most bacteria free living, 10-20% attached to particles. Bacteria consume 10-50% of primary production, as DOM. Controls on bacterial numbers: C, N, or other nutrient limitation, Grazing by nanoplanktonic heterotrophic grazers,

(small flagellates, 3-10 M, ~1 day doubling time).Carbon and energy returned to main food chain inefficiently. Are bacteria the main source of remineralization of nutrients? Do bacteria compete with phytoplankton for nutrients?

Ducklow et al. (1986) released 14C-labeled glucose into a 300 cubic meter mesocosm in coastal waters, followed the labeled carbon for 50 days: High growth efficiency for bacteria, can be > 50%.

Bacteria can comprise > 20% of POC.Glucose rapidly taken up by bacteria (< 1 day). Only 2% of labeled Carbon transferred to larger organisms 20% ended up in POC, majority respired by bacteria, mostly in < 1 M. Bacteria largely a sink for organic carbon and source of remineralized nutrients.

Sherr & Sherr (1988) argued that the microbial loop organisms are intimately linked with the smaller (~ < 5 M) phytoplankton. This includes the picoplankton and smaller nanoplankton. While this microbial food web may only inefficiently pass POC to the metazoans (macrozooplankton and higher trophic levels), it still may be the main source of POC in nutrient limited systems where large phytoplankton are scarce. The Microbial Loop refers to the cycling (and recycling) of elements between inorganic nutrient pools, smaller phytoplankton, dissolved organic pools, bacteria, and the microzooplankton. Some materials flow through the microbial loop in all marine systems. In low nutrient regions like the mid-ocean gyres, most carbon from photosynthesis cycles through the microbial loop. In nutrient rich systems (coastal upwelling zones, during the spring bloom in the North Atlantic), > 50% of carbon may follow the classic ocean food chain.

Phytoplankton (large)

(all)

DOM

Heterotrophic Bacteria

Macrozooplankton

Microzooplankton

Flagellates

Small Fish

The Microbial Loop (Pomeroy, 1974; Azam et al., 1983)

Larger Fish, Higher Trophic Levels

Classic Ocean Food Chain

(from Sherr & Sherr, 1988)

The efficiency of the biological pump out of surface waters is a function of the biological community. A community with large phytoplankton species and abundant large copepod grazers will export organic matter out of the euphotic zone efficiently (high f-ratio or export ratio). Most carbon into classic food chain. A community dominated by picoplankton and nanoplankton with microzooplankton doing most of the grazing will not export organic material efficiently, most matter will be recycled within the euphotic zone (low f-ratio or export ratio). Most carbon into the microbial loop. The community in turn is largely driven by new nutrient inputs.