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Marine Ecosystems and Nutrient Cycles

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Marine Ecosystems and Nutrient Cycles Exercise " 18 Marine Ecosystems and Nutrient Cycles - -" ~~~l~~ OBJECTIVES: of energy may be represented by a trophic pyra­ mid (Figure 18-2) or trophic web (Figures 18-3 1/1 To understand the role of the ocean as an ecosys­ and 18-4). The second property is that individual tem and nutrient recycler. nutrients, elements necessary for metabolic II To understand the interactions between and flow processes, are recycled many times within most ,· ' 0" of energy through producers, consumers, and de­ ecosystems, with decomposers playing a crucial f~~' composers. role in the release of nutrients from organic matter. II To appreciate how hwnans can disrupt marine ecosystems. Trophic Pyramids and Webs: Examples from the Antarctic ';Ill this exercise we explore how energy from pri­ Ocean mary productivity (Exercise 16) is transferred through an ecosystem. An ecosystem consists of A simplified trophic pyramid for the Antarctic a group of Jiving organisms, the physical environ­ Ocean is presented in Figure 18-2. Diatoms are the ment in which theylive, and an energy source (e.g., primary producers, providing energy for the entire sunlight .in photosynthesis-based ecosystems). The ecosystem, and are shown at the base of the pyra­ largest ecosystem can be considered the earth as a mid. These primary producers are consumed by whole; the planet may be subdivided into terres­ the primary conswners (herbivores) of the trophic , -, ~..~~-. trial and marine ecosystems, and each of these may pyramid's s~cond tier, mostly krill. In turn, krill are be further subdivided, often on the basis of envi­ the energy source for the third trophic tier, the ronmental conditions (e.g., depth, temperature, whales. The whales are termed secondary con- , ;. ~~ '~ ,",' etc.) , In each ecosystem, there are organisms that ::~~~,~h(;h~~'~::'~ ~~~=~:~s~~ ~f~ ~ _-~ produce food (primary producers or autotrophs), first-level organisms that consume other organisms (sec­ In Figure 18-3, a more realistic model of en­ ondary producers or heterotrophs), and organ­ ergy .flow through the Antarctic Ocean ecosystem is isms that decompose autotroph and heterotroph presented. This trophic web is essentially a trophic waste products and bodies after death (decom­ pyramid expanded to include the more complex posers; generally fungi and bacteria). Ecosystems interrelationships between organisms at higher - - .:.~ ~ " . .. --";-,' : - ' - ~' ,. have two fundamental properties (Figure 18-1). trophic levels; the trophic relationship between the "'"'-- j - -_:- .~. ~.~ , _- I The first is that energy flows through an ecosystem diatoms (primary producers) and krill (secondary .:.. ':~ .;'- "I in only one direction: it is received from the sun, consumers or herbivores) rema.ins the same. Note , I -x, transformed into organically usable forms through that there is a greater diversity of organisms at the . ... .. ,1 primary producers, and flows to secondary pro­ higher trophic levels and that some of these can .ducers and decomposers. Depending upon the operate at multiple trophic levels. In this trophic :-,:- :. -:~ : :'.,:omplexity of the ecosystem, this one-way transfer web, blue whales remain in the third trophic level, . ~ :.: : : ' ~ ,. .:~-.: ~.' .::: .,.. - .- : :~ ..:.~.~: ~~ 185- ""-t " "< " - ~ -,. " . ~:. ".-.: ~-:... ~:::- "".s::::"..... 186 Marine Ecosystems and Nutrient Cycles Primary producers i I I I -+--------- Energy Energyand nutrients Nutrientsonly Figure 18-1 Generalized ecosystem diagram showing the one-way flow of energy and the recycling of nutrients. Note that nutrient recycling is not "perfect"; some organic mat­ ter sinks out of the photi c zone before it is broken down by decomposers, and some frac­ tion of this amount is buried (not shown). but other organisms directly dependent upon krill Compare the Antarctic food web to that of th( , . ;. ,~:~:.~.; j are also included (i.e., crabeater seals, winged Long Island estuary (Figure 18-4). Notice that birds, Adelie penguins, and small fish and squid). trophic levels in the estuary increase from left to The small fish and squid, in turn, are prey items for right, from primary producers (plants, phyto­ emperor penguins, larger fish, and Weddell and plankton) to top carnivores (birds). Such complex­ Ross seals-members of the fourth trophic level. ity is characteristic of many marine food webs, Because skuas feed upon the chicks of Adelie pen­ with many interrelations between organisms. guins, they are also considered members of the Ecological theory states that the greater the num­ fourth trophic level. The remaining organisms in ber of food pathways leading from the primary the trophic web, the leopard seals and killer whales, producers to higher trophic levels, the more resis­ occupy multiple trophic levels, as they feed upon tant the ecosystem will be to disturbances related multiple trophic levels below them. For example, to losses of individual species. Why might this leopard seals may be assigned to the fourth trophic be so? level when feeding upon crabeater seals or Adelie penguins, or the fifth trophic level when feeding upon emperor penguins, large fish, or Weddell and Ecological Efficiency and Biological Ross seals. Similarly, the killer whale belongs to the Magnification fourth trophic level when feeding upon blue whales or crabeater seals, or to the fifth trophic Energy contained within an ecosystem is not recy­ level when feeding upon leopard seals. Note that cled, but moves unidirectionally through succes­ some organisms may shift their trophic position sively higher trophic levels. Within a given trophic during their lifetime; for example, as fish grow level, the vast majority of energy obtained from the larger, some shift from the second to third trophic level below is used for respiration and metabolism level. Appreciate that 13 different species are di­ and is lost as excretion or heat; only a small po~ .: •. rectly dependent upon krill, which are themselves tion is transformed into biomass through growth. dependent upon diatoms. In addition, not all available individuals in the Exercise 18 187 lower trophic level will be consumed; some will die tem is in converting solar energy into food ,prod­ of natural causes. Both of these factors result in a ucts for humans. phenomenon termed ecological efficiency, which Related to this concept of ecological efficiency expresses the amount of energy (biomass) flowing is the biological concentration of pesticides, such into a trophic level compared to the amount of en­ as DDT, in the marine ecosystem. DDT is an ex­ ergy retained within that trophic level. In most tremely effective agent against agriculture-damag­ ecosystems, ecological efficiency is less than 10 per­ ing insects. and has been in extensive use since the cent. For example, in Figure 18-2, the numbers on 1940s, although banned in the United States since the left indicate that of the roughly 1000 biomass 1972. One reason the pesticide is so effective is its units of the first trophic level, only 100 units (- 10 high resistance to biological breakdown. This resis­ percent) of this energy is converted into biomass at tance leads to DDT's eventual transport from agri­ · 1 ., I the second trophic level. Another way to visualize cultural fields through irrigation and runoff into ': 1 this relationship is that 100 grams of diatoms are the ocean, where it is incorporated into the marine required to support 10 grams of krill, and 10 biosphere during primary production. Like most grams of krill are required to support I gram of pesticides, DDT is damaging to non-insects as well; blue whale. From a fishery standpoint, the greater in phytoplankton, it decreases the efficiency of the number of trophic levels between primary pro­ photosynthesis and therefore reduces the total pri­ ducers and edible fish, the less efficient the ecosys- mary production supporting higher trophic levels. 1 ! 10------------------------------------- Figure 18-2 A simplified trophic pyramid for the Antarctic ocean. [After Robert C. Murphy, "The Oceanic Life of the Antarctic ." Copyright © 1962 by Scientific American, Inc. All rights re­ served.l 188 Marine Ecosystems and Nutrient Cycles .... Krill ------ . ~ ~--.:::::::::::::::lE".,c~!L f;,, ;~~~~ . "'~:' ~ ~ ..- f. ';l.::tl:. i ;.;. t . ~... ... ... ... ... Blue Whale ...,, \ \, I I ) ~"r ~----------------------------~~:/; ~ .; " »" ..."7' W'mge d birr d s ,I I" / Adelie penquln ~ ,,1 (" :" ... -------- ~Q ~ ~-_-_-_-_-_- .;/f I " .... -3"' . I I, /" ., ..' Small fish I I' " squid ."~~. - ~ 54 Empe'o' penguin _ - _-- , .... ---------------- ~ ,I ~ ·-----------------::,,1 I I Large fish / : I lJ<_~ I ... ': ~..- I I I , I Weddell seal II I II I ,,/ I ~--------------~ I : ... I I' , I ( Ross seal / I, ,, ,~------>~~-----------_ ... .: ... ... , ... I Leopard seal , I ft. , I ... I ...... I ... I " ...... ,... .... :-----2":::=._----- Figure 18-3 Summary of major trophic relation ships with in t he Ant arctic ecosystem. [Aft er Robert C. Murphy, "The Oceanic Life of the Antarct ic." Copyright © 1962 by Scientific American, Inc. All rights reserved.] Exercise 18 189 .~ ~n 3.15, 5.17, 4 ~75, 6.40 Organic debris Marsh 13 pounds per acre ~.~~ r Bottom 0.3 pound per acre I ... Y -;J , L I ~..~, 1.......... I Billfish 2.07 I I Osprey (egg) 13.8 Green heron 3.57,3.51 Water plants 0.08 f i Marsh plants f/!l: Shoots 0.33 ~~Z C!!lilri:.* Minnow 1.24 ---------.---+- ENERGY FLOW Figure 18-4 Summary of major trophic relat ionships within the Long Island estuary. The numbers beside each organism's
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