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Chapter 2.1 Structure & Productivity of Aquatic Ecosystems • Aquatic Ecosystems consist of entire drainage basins. • The nutrient and organic-matter content of drainage from the catchment area is modified in each of the terrestrial, stream, and wetlandlittoral components, as well as in the lake or reservoir per se. • Productivity is generally low to intermediate in the terrestrial components, highest in the wetland interface region between land and water, and lowest in the open-water portion of the lake. When we talk about "production" we are talking about either primary production, secondary production, or tertiary production. Primary production processes deal with photosynthesis, that is the creation of plant biomass from carbon dioxide, water, and radiant energy. Secondary production is the production of small animals that feed on the primary producers. Tertiary production is the production of larger fish and also animals that feed on the lower trophic level (secondary producers). Primary Production • amount of plant tissue built up by process of photosynthesis over time is primary production • majority of primary production is accomplished by small, dispersed pelagic phytoplankton rather than plants (plants only account for 5-10% of total marine productivity) • the total amount of organic material produced by photosynthesis represents gross primary production • as a portion of the organic material produced by photosynthesis is utilized in cellular respiration, any excess production is referred to as net primary production; represents amount of organic material available to support consumers, nonphotosynthetic protists, and decomposers • standing stock refers to the number of organisms per unit area or per unit volume of water at the moment of sampling. • biomass is defined as the total weight (total numbers X average weight) of all organisms in a given area or volume; this measure is used in preference to standing stock because phytoplankton vary in size and total numbers of all organisms are not as ecologically meaningful as estimates of their biomass Measurement of Primary Production: The Light and Dark Bottle Method • • A sample of water containing plankton is placed in a transparent (clear) bottle and placed in the site for one day; oxygen is produced by the plants and is also consumed by the plants and animals A similar sample of water is also placed in a dark bottle (so no sunlight can get in, usually a bottle painted black); if the bottle is black no sunlight can enter the bottle and thus any change in oxygen is due to respiration (due to plants and animals). Transparent Bottle Opaque Bottle Initial, 6AM 0.288 0.288 Final, 9AM 0.292 0.282 Difference +0.004 -0.006 Gross Productivity 0.010 millimoles O2/liter Net Productivity 0.004 millimoles O2/liter The clear bottom picked up +0.004 (photosynthesis + respiration) but the dark bottom (respiration only) lost 0.006. We can calculate the "Gross Primary Productivity" as the difference between the final bottle and black bottle at 9 PM = 0.010 millimoles O2/liter/day. And, we can calculate the "Net Primary Productivity" as the difference between the final bottle at 9 PM and the black bottle at 6 AM= 0.004 millimoles O2/liter/day. Dissolved Oxygen (DO) profiles In addition to using light and dark bottles to characterize aquatic metabolism, we can use DO probes to characterize changes in oxygen with depth. In lakes that are deep, this could be interesting for detecting the zone where the photosynthesis : respiration ratio is low, and the lake flips from a net autotrophic to net heterotrophic system (See figure) Factors affecting Primary Productivity: Light • Amount of light varies with latitude; it decreases from the equator towards the poles – polar regions: a single pulse of phytoplankton abundance occurs during the summer when light becomes sufficient for a net increase in primary productivity – temperate latitudes: primary productivity is generally maximal in spring and autumn when the combination of available light and high nutrient concentrations allow plankton blooms to occur – tropics: intense surface heating produces a permanent thermocline so phytoplankton are nutrientlimited year round and there are only small fluctuations in primary productivity Factors affecting Primary Productivity : • Nutrients nutrients: major inorganic nutrients that are required by phytoplankton are nitrogen and phosphorus (diatoms and dinoflagellates also required silica); all of these nutrients occur in small amounts and are thus limiting factors for primary productivity; each species of phytoplankton has a particular response to different concentrations of limiting nutrients and each has a maximum growth rate – – – oligotrophic regions have low concentrations of essential nutrients and therefore low productivity eutrophic waters contain high nutrients and support high numbers of phytoplankton mesotrophic waters have nutrient levels between those of the two extremes Conclusions Aquatic ecosystems provide a wide range of benefits to people. As efforts to improve water productivity intensify, there is growing recognition that future investments in water management need to consider how to sustain these ecosystems and the benefits they provide. In order to maintain the productivity of aquatic ecosystems and improve dependent livelihoods, policies, institutions and governance arrangements that foster sustainable and equitable use of these resources need to be developed. In particular, the full value of the range of aquatic ecosystems within different river basins, the resources they provide, and the trade-offs among different uses, need to be better understood; the environmental flows required to sustain aquatic ecosystems and their values need to be quantified; and ways to improve water productivity by incorporating aquatic ecosystems need to be identified.