User:Chadwongchong

The Effects of Mesopelagic Heterotrophic Zooplankton on Carbon Export
1 Mesopelagic Zooplankton

1.1 Mesopelagic Zooplankton

The microbial group of mesopelagic zooplankton includes a broad categorization of heterotrophic marine microbes that exist at depths between 200m and 1000m below sea level. The mesopelagic zone, also known as the twilight zone is succeptable to a scarce amount of light that is only able to penetrate the upper 200m of the zone itself. This amount of light is insufficient for the process of photosynthesis to occur making it difficult for photoautotrophic organisms to exist, thus most of the organisms in this region are heterotrophic; primarily relying on other organisms as a source of nutrients to maintain their daily requirements for internal metabolic processes and reproduction. These organisms are able to consume various forms of nutrients, such as in the form of small faecal pellets and marine snow. Since these organisms are completely dependent on viable organic nutrients from within the water column, they have developed the ability to migrate vertically to optimal depths at which hot spots of organisms and food particles exist. This ability allows them to maximize their efficiency and minimize the amount of energy used during feeding (Gorsky and Gaard, unknown), a biological function known as diel vertical migration. Additionally, this ability allows them to move to optimal depths during certain times of the day to reduce their predator encounter rate. In having this ability to migrate vertically, they prove to be a vital component in the process of carbon export as they are able to consume various forms of organic carbon in the ocean and consequently produce large, dense faecal pellets that sink at relatively high rates. In having this ability to consume small suspended particles as well as the ability to consume other zooplankton, a large fraction of the ocean's exported organic carbon can be attributed to the existence of mesopelagic zooplankton.

2 Consumption of Organic Carbon

2.1 Faecal Pellets

Various forms of nutrition are available to mesopelagic zooplankton; faecal pellets being one of the most plentiful and accessible. These sinking pellets are the metabolic excrements of other marine organisms and are found in various shapes, sizes and colours depending on the producer of these pellets. Upon close examination of these nutritious pellets, a large portion consists of undigested organic material containing a plentiful amount of viable nutrients. This includes the reminants of plant cells, exoskeletal materials and various organic matter, all of which vary from organism to organism. These faecal pellets are first intercepted by the zooplankton during its sinking process, and then is subsequently ingest for digestion and assimilation. These pellets are ingested into the gut in as large a quantity as possible, where the food is then subject to abrasion, deterioration by pH levels and the action of enzymes. However, some material may be more difficult to digest that others such as the frustules of coccolithophorids, which contains a source of pectin that is resistant to enzymatic digestion. The labile compounds are then absorbed through the walls of the lining of the gut and all of the unabsorbed organic matter is passed into the gut lumen and out in the form of faecal pellets once again. As this particulate matter exits the digestive process of the zooplankton, it will either contribute to the flux of carbon or will remain buoyant at the depth of excretion; both of which will contribute to the recycling of organic carbon through the microbial food loop whereby other organisms will use them for its remaining nutritional value.

2.2 Marine Snow

Marine snow is a source of organic material found in a cluster of primarily surface-generated particulate matter settling to the depths of the ocean. This matter is bound together by an aggregate mucus composed of transparent exopolymer particles, which are naturally generated by marine bacteria and plankton. Its composition is high in organic nutrients making it a viable source of energy for mesopelagic zooplankton. According to a study by Lampitt et al. (1993), the marine snow collected in the North East Atlantic is a matrix of cyanobacteria and photosynthetic picoplankton with a smaller fraction comsisting of crustacean moults, faecal pellets, pennate diatoms, diatom flocs, faecal aggregates, larvacean houses and pteropod feeding webs. The specific composition of marine snow will vary spatially and temporally, however all forms will possess nutritional value to its consumers. This organic aggregate is usually ingested by way of filter-feeding, but in most cases particles smaller than ~5um in diameter are too small to capture (Frost 1972). Therefore, by incorporating a cluter of small particles into marine snow aggregates, zooplankton have the ability to consume them. Once ingested, the aggregate's nutrients are absorbed through the gut and the excess unabsorbed material is excreted in the form of faecal pellets, thus recycling the detritus and contributing to the flux of organic carbon to greater depths of the ocean. By having the ability to consume such a variety of forms of organic carbon, the ecologic role of these mesopelagic zooplankton is key to the export of carbon to the deep ocean and the recycling of sinking particulate matter.

2.3 Diel Vertical Migration

The ability of mesopelagic zooplankton to move spatially in response to ecological gradient forcing is key to their existence and to the flux of carbon. This movement pattern is generally controlled a variety of factors; phototaxis, geotaxis, temperature and diurnal internal periodicity (Eppley et al. 1968). However, one of the most influential factors in vertical migration is the nutrient availability. Acoustic imaging data suggests that a significant amount of particulate matter exists in the mesopelagic zone and it is believed that this surplus of organic carbon is the driving factor for these zooplankton (Gorsky and Gaard, unknown). Therefore, these organisms are able to migrate vertically to these optimal depths of particulate matter, which increases their metabolic efficiency and increases the amount of carbon that is recycled through this microbial group. A study conducted by Lampitt et al. (1993) investigated the distribution of marine snow in the NE Atlantic Ocean. The results of the experiment found that although variations in marine snow abundance was sometimes large, the subsurface maximum was usually between 40m and 80m in depth. Figure 1 illustrates the distribution of marine snow found during the expedition.


 * NOTE: If the image link is broken, Figure 1 can be viewed at http://s74.photobucket.com/albums/i250/FrustratedMunky/?action=view&current=Figure1MarineSnowAbundance.jpg

As illustrated in Figure 1, depth at which marine snow abundance is at its highest is above the mesopelagic zone. Therefore, these zooplankton must migrate to relatively shallow depths in order to feed under optimal conditions; sometimes up to 160m above the pelagic zone in order to reach optimal feeding depths. Generally speaking, these mesopelagic zooplankton exist in depths where light is very limited making predation less of an issue. However, since nutrition is such a strong influential factor in the life of these organisms, they are forced to migrate to such shallow depths where light is much more available. As a result, these organisms are often under conditions of strong predation at times when they in an optimal feeding environment making for a difficult decision: does the natural response to predation override the importance of nutrition? Many experiments have looked at the effects of predator avoidance behaviour on diel vertical migration (Bollens and Frost 1991, Neill 1990, Lampert 1989) however none of which have looked at the predatory factor against that of nutrient availability. Yet it must be acknowledged that marine show remains available at depths in the mesopelagic zone; marine snow will decrease exponentially from the depth at which the maximum abundance is found. Therefore it is not completely necessary for zooplankton to venture to such shallow depths to obtain their food as there is available nutrients in the form of faecal pellets and sinking detritus that is readily available in the mesopelagic zone.

3 The Export of Carbon

3.1 Carbon Flux

The ability for mesopelagic zooplankton to consume such a vast variety of particulate matter is a predecessor to the amount of carbon that is exported from this region. In the process of consuming and digesting sinking detritus and small particulate matter, they are unknowingly providing the service of repackaging organic fragments into high density, fast-sinking faecal pellets. Standard faecal pellets are much more succeptible to natural deteriorative processes; often fragmenting into slower or non-sinking particles by processes such as sloppy feeding or natural abrasion where they are then reingested by smaller marine microbes or become part of the aggregates of marine snow (Limpitt et al. 1990). However, the large faecal pellets produced by these zooplankton are high in density and prove to be highly efficient modes of carbon flux to deep ocean waters, providing organisms of the benthic zone with a source of organic nutrients. Moreover, the amount and type of food that is available to these zooplankton will determine the composition of the pellets themselves. For instance, under conditions where nutrient availability is high, larger faecal pellets will be produced; since there is a surplus amount of carbon, the ingested food will pass through the gut tract much faster than usual and the zooplankton will spend less time assimilating its nutrients. As a result, the excess material is expelled in the form of larger faecal pellets that sink significantly faster than smaller pellets and consequently contribute more to the vertical flux of organic carbon. As zooplankton have the ability to consume a variety of sources of organic carbon, their choice of prey will also determine the size of the pellets; herbivorous zooplankton will feed on phytoplankton or protozoans that may have hard housings that are difficult to digest thus resulting in larger faecal pellet sizes, whereas the consumption of plankton with less resistant structures will result in smaller, more compact faecal pellets. In addition to the importance of the nutrient source, the physical properties of the faecal pellets themselves are key in determining the sinking rate and the nutritional value of the excrements. A study by Ferrante and Parker (1977) establishes that the sinking rates of faecal pellets increase as the shape of the pellets replicate a cylindrical and tapered form as a result of decreasing resistance. The research also provides evidence of zooplankton faecal pellets sinking at rates between <10 -100 meters/day in comparison to sinking phytoplankton cells whose rates range between <1 - 10 meters/day. Therefore, the production of faecal pellets by marine zooplankton will generally result in an increase in carbon flux rate. This carbon will then proceed down the water column and may be reassimilated by other marine microbes or it may reach its destination in the aphotic zone where it will provide as a source of nutrients to bottom dwelling organisms and will become sequestered in the deep ocean sediments.

3.2 Influential Factors on the Quantity of Carbon Flux

The metabolic processes governed by mesopelagic zooplankton are clearly one of the largest influential factors in the amount of carbon that is exported in our ocean waters. In the study by Wilson et al. (2005), the analysis of faecal pellet characteristics was used to quantify the extent at which particulate matter is repackaged by mesopelagic zooplankton. Two regions were analysed; one of which was located in a mesotrophic environment (Japanese time-series station K2) and another in an oligotrophic environment (Hawaii Ocean Time series station ALOHA). As a result of its findings, mesopelagic zooplankton faecal pellets contributed to 14-35% and 3-39% of the particulate organic carbon (POC) flux through the mesopelagic zone at ALOHA and K2 respectively; variability in percentages were a result of the variability in sampling locations and the season in which sediment trapping was done. These results support the fact that mesopelagic zooplankton play a large part in the repackaging and the rate of carbon export from the surface region of the ocean to deep ocean waters. However, there are a considerable amount of factors that cannot be controlled by the zooplankton themselves. In such a nutrient-limited environment as the mesopelagic zone, the accessibility to nutrition is completely dependent on their proximity to organic carbon, which is subsequently dependent on the amount of biomass that exists in the region. Therefore in areas of high productivity, there will be larger flux rates of marine snow, particulate matter and sinking detritus. Regions below such productive areas will naturally attract zooplankton based on the surplus amount of nutrient abundance. The general biomass of the marine community will be the basis for zooplankton nutrition, whether it be in the form of living photoautotrophic phytoplankton, smaller heterotrophic zooplankton or suspended particles, all of which are directly correlated to the productivity of the region its ecological conditions.

4 References


 * Bollens, Stephen M. And Frost, Bruce W. “Diel Vertical Migration in Zooplankton: Rapid Individual Response to Predators.” Journal of Plankton Research 16.3 (1991) : 1359-1365


 * Forward Jr., Richard B. Diel Vertical Migration: Zooplankton Photobiology and Behaviour. Scotland: Abereen University Press, 1988.


 * Wilson, Stephanie E. et al. “Changes in Fecal Pellet Characteristics With Depth as Indicators of Zooplankton Repackaging of Particles in the Mesopelagic Zone of the Subtropical and Subarctic North Pacific Ocean.” Virginia Institude of Marine Science 2894


 * Wotton, Roger S. And Malmqvist, Bjorn. “Feces in Aquatic Ecosystems.” Bioscience 51.7 (2001) : 537-544


 * Shanks, Alan L. And Trent, Jonathan D. “Marine Snow: Sinking Rates and Potential Role in Vertical Flux.” Deep-Sea Research 27A (1980) : 137-143


 * Lampitt, R.S et al. “Marine Snow Studies in the Northeast Atlantic Ocean: Distribution, Composition and Role as a Food Source for Migrating Plankton.” Marine Biology 116 (1993) : 689-702


 * Legendre, Louis and le Fèvre, Jacques. “Microbial Food Webs and the Export of Biogenic Carbon in Oceans.” Aquatic Microbial Ecology 9 (1995) : 69-77


 * Steinberg, Deborah K. et al. “Role of Mesopelagic Zooplankton in the Community Metabolism of Giant Larvacean House Detritus in Monterey Bay, California, USA.” Marine Ecology Progress Series 147 (1997) : 167-179


 * Richardson, Tammi L. And Jackson, George A. “Small Phytoplankton and Carbon Export from the Surface Ocean.” <U>Science</U> 315 (2007) : 838-840


 * Heiskanen, Anna-Stiina. “Contamination of Sediment Trap Fluxes by Vertically Migrating Phototrophic Micro-organisms in the Coastal Baltic Sea.” <U>Marine Ecology Progress Series</U> 122 (1995): 45-58


 * Zooplankton Fecal Pellet Guide. University of Massachusetts. 12 April 2009 <http://www.zfpguide.com/learn-more.aspx>


 * Turner, Jefferson T. and Ferrante, John G. “Zooplankton Fecal Pellets in Aquatic Ecosystems.” <U>Bioscience</U> 29.11 (1979): 670-677