Exercise " 18 Marine and Nutrient Cycles

- -"

~~~l~~ OBJECTIVES: of 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 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 (Exercise 16) is transferred through an . 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. are the ment in which theylive, and an energy source (e.g., primary producers, providing energy for the entire sunlight .in -based ecosystems). The ecosystem, and are shown at the base of the pyra­ largest ecosystem can be considered the as a mid. These primary producers are consumed by whole; the planet may be subdivided into terres­ the primary conswners () 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 (primary producers or ), first-level organisms that consume other organisms (sec­ In Figure 18-3, a more realistic model of en­ ondary producers or ), and organ­ ergy .flow through the Antarctic Ocean ecosystem is isms that decompose and presented. This trophic web is essentially a trophic waste products and bodies after (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 , . ~ :.: :

: ' ~ ,.

.:~-.: ~.' .::: .,.. - .-: :~ ..:.~.~: ~~ 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 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 (, phyto­ emperor penguins, larger fish, and Weddell and ) to top (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 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 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 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 . 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 during . 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 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

------.---+-

Figure 18-4 Summary of major trophic relat ionships within the Long Island estuary. The numbers beside each organism's name refer to the average DDT content, in parts per mil­ lion , found in each organism . [After George M. Woodwe II, "Toxic Substances and Ecological Cycles." Copyright © 1967 by Scientific American, Inc. All right s reserved.l

Unfortunately, the effects of DDT on the marine mass and becomes increasingly concentrated at ecosystem do not end at the first trophic level. higher trophic levels, becoming the highest in the Because it is not easily broken down, the pesticide top carnivores (Figure 18-5). is stored in body tissue and transferred up through This phenomenon is called biological magnifi­ higher trophic levels. When DDT-enriched phyto­ cation, and the resulting DDT concentrations can plankton are consumed by primary consumers, the wreak havoc with biological processes ranging 10 percent efficiency effect applies to organic com­ from eggshell calcification in birds to reproduction pounds, but not to DDT. Thus, the DDT is pas­ in humans. For example, in the final years before sively transferred through the ecosystem. While the 1972 U.S. ban on DDT, brown pelicans, which some DDT is given off through respiration and ex­ nest on islands off the coast of southern California, cretion at each level, the bulk remains in the bio- had acquired so much DDT that most of their 190 Marine Ecosystems and Nutrient Cycles

likely have some amount of DDT in your body . right now, and it will only increase during youi' lifetime. Good .news? No. A fact? Yes. . o Biomass Carnivore2 Losseslhrough ( respiration excretion Biogeochemical Cycles

. . , DDT The main factor limiting productivity in well-lit marine waters is the availability of principal nutri­ :'. ents such as nitrate, phosphate, and (al­ though carbon is not limiting in most regions). . , ... " .. Here we will examine the cycling of these nutrients J ~ through various "reservoirs" in the , ·". . lithosphere, , and biosphere, and see · " how their distribution and exchange affects the 1 .. :'. " marine ecosystem, The general biogeochemical cy­ ',., cle involves the intake of an inorganic form of a · " nutrient by an autotroph during photosynthesis of organic molecules, which are subsequently trans­ .. ferred through the trophic web by heterotrophic

, , activity. Eventually; decomposers transform dead organic matter and waste back into its inorganic form, making the constituent nutrients available again to autotrophs. Keep in mind that decom­ H '~ posers, such as bacteria and fungi, are critical com-: ponents in recycling such nutrients. '. ). . Carbon is the basic building block of all organic molecules. Its is presented in Figure 18-6. Note that carbon is rarely limiting in most marine ecosystems; only about 1 percent of the carbon reservoir in the ocean-is involved in primary productivity at any given time, Another major carbon reservoir is at­

mospheric carbon dioxide (C02), which enters the ocean by gas exchange at the air-water interface. Figure 18-5 Biological magnification of DDT in a ma­ Additional carbon dioxide comes from the respira­ rine . [After George M, Woodwell, "Toxic tion of plants and animals. Thus, dissolved carbon Substances and Ecological Cycles," Copyright © 1967 by dioxide is readily available to autotrophs for pho­ Scientific American, Inc. All rights reserved.l tosynthesis, which «fixes" carbon into organic mol­ ecules that are then transferred through the trophic web by and feeding. Carbon is eggshells would break when incubated, causing the also present in carbonate sediments and limestones pelican population to drop precipitately. Since the composed of the calcareous skeletons of marine 1972 ban, the pelican population has recovered, organisms, and as natural petroleum deposits (i.e., but DDT concentrations remain relatively high. oil, gas, peat, ) formed by burial of ancient or­ Concentrated levels of DDT have also been found ganic material. Carbon within these reservoirs in Adelie penguins and skuas, which indicates that tends to have a long residence time (the average the pesticide has been spread through oceanic cir­ time interval that individual particles remain in a. culation and other processes to regions far re­ given reservoir), but is ultimately cycled through' moved from its original application. You most terrestrial weathering in dissolved form to the Exercise 18 191

" ~ i I I

Figure 18-6 The biogeochemical cycle of carbon in organic and inorganic forms. [After Harold V. Thurman, "Essentials of Ocean ography." Copyright © 1983 by Charles E. Merrill Publishing Co. All rights reserved .]

Denitrifying d7l ~ "", bacteria bacteria -f'J~ ~:,:-¢t?J~

~ Bacteria ~ Ammonium ------: J. ~\" .~

~, Nitrite -Nitrous ~acteria oxides

- Organic nitrogen compounds _Inorganic nitrogen compounds

Figure 18-7 The biogeochemical cycle of nitrogen in organic and inorganic forms. [After Harold V. Thurman, "Essentials of Oceanography." Copyright © 1983 by Charles E. Merrill Publishing Co.All rights reserved .l

ocean or oxidized by natural or human-related blocks of proteins within all living things. The bio­ burning into the atmosphere. geochemical cycle of nitrogen is summarized in Figure 18-7 and involves a fairly comple: suite of . Nitrogen is a essential element bacterial "fixers" and decomposers. Molecular ni­

in the production of amino acids-the building trogen (N 2) cannot be used directly by most 192 Marine Ecosystems and Nutrient Cycles organisms, but metabolic processes within cyanobac­ nutrient-rich bottom waters. Some denitrifying..--... teria convert dissolved nitrogen into nitrate (NO j ) bacteria will completely oxidize the nitrogen-bear/ . ' through a process called . The re­ ing compounds back into molecular nitrogen," sulting nitrate is the nutrient form most easily uti­ some of which may be exchanged with the atmos­ lized by most phytoplankton and is subsequently phere. In addition, small amounts of nitrogen are transferred up the trophic web through feeding. buried in ocean sediments and released through Excreted waste and dead organic matter are terrestrial weathering-for simplicity and because broken down by decomposers as an energy source. of their relatively small contribution to the marine Some of these decomposers are denitrifying bac­ cycle, these paths are not shown in Figure 18-7. teria, whose metabolism breaks down organic­ bound nitrogen into progressively oxidized inor­ Cycle. Phosphorus is an essential ganic forms: the first is ammonia (NH3), followed element in all living organisms' genetic informa­ by nitrite (N02), or finally nitrate (N03). The less tion (DNA and RNA) and in the ATP compounds oxidized forms (ammonia, nitrite) are generally involved in the conversion of carbohydrates into taken up again by autotrophs before denitrifying energy. Its biogeochemical cycle is simpler than bacteria can fully oxidize the compounds to ni­ that of nitrogen, largely because the bacterial com­ trate. Only a small percentage of nitrogen initially ponent of the cycle is simpler (Figure 18-8). fixed by cyanobacteria is recycled within the photic Phosphorus is released to the ocean through the zone; the majority is not oxidized back into a us­ weathering of phosphate-bearing rock and re­ able form until it is well below the photic zone and moved through burial of organic matter (not thus unavailable to photosynthesizing autorrophs, shown). Inorganic orthophosphate may be used The major mechanisms for returning this fixed ni­ directly by autotrophs and, like nitrogen, is trans­ trogen to the photic zone are seasonal vertical mix­ ferred to higher trophic levels through feeding. The ing of the and local upwelling of phosphorus within excreted waste and dead or-

i ~ , ' ' : --" ."., Dead plant tissue ~

?~ ~' ~ ~sue ~. . : .:...J . Deadf'~ Deaq, plant \ anlm~1 tissue tissue Usable I , phosphates

Guano and ..:bone remains \~~LJJ Phosphates freed by weathering and erosion

- Organic phosphorus Mechanical and autolytic compounds /"release of phosphates ~ _ Inorganic phosphorus compounds

Figure 18-8 The biogeochemical cycle of phosphorus in organic and inorganic forms. [After Harold V. Thurman, "Essentials of Oceanography." Copyright © 1983 by Charles E. Merrill Publishing Co. All rights ressrved.l Exercise 18 193

ganic matter is released back into the environment biomass through primary productivity. In addi­ through several pathways-all much faster than tion, significant amounts of nutrients are exported those for nitrogen. As a result, phosphorus is a less from the photic zone through the continuous or­ limiting nutrient in the photic zone, even though ganic "rain" of fecal material, carcasses, and molts. its concentration is roughly one-seventh that of ni­ while bacterial of this organic trogen. As with nitrogen, some phosphorus may be "rain" continues beneath the photic zone, the re­ deposited in ocean sediments or in coastal regions leased nutrients are effectively sequestered from in the form of bird guano; these materials are the photic zone by the strong density difference eventually recycled through weathering and ero­ between the mixed layer and the deep-water layer, sion back into the ocean. as marked by the thermocline. Thus, if vigorous productivity is to take place, these nutrient-rich deep waters must somehow be transported up­ Nutrient Distribution in the Ocean ward to "fertilize" the nutrient-poor photic zone. Physical mechanisms that accomplish this fertiliza­ The availability of nitrogen and phosphorus often tion include upwelling and the weakening of the limits primary productivity and the resulting thermocline during the winter season or intense photic zone biomass. As shown in Table 18-1, the storms. Note that the absolute concentrations of open ocean constitutes roughly 90 percent of the dissolved nitrogen and phosphorus are higher in total oceanic environment and is often considered the deep waters of the Pacific and Indian oceans a "biological desert" because of the paucity of dis­ than in the Atlantic. This is because bottom waters solved nutrients within the photic zone. Typical in the Pacific and Indian oceans have been in the vertical concentrations of dissolved nitrate and thermohaline circulation system for a longer time phosphate for different oceans are shown in and have thereby accumulated more dissolved nu­ Figures 18-9. These nutrient patterns are the direct trients from overlying surface waters. . result of the biogeochemical cycling discussed Oceanographers often construct "box models" above. Dissolved nutrient concentrations are low to summarize how nutrients are cycled through in the photic zone because available nitrogen and different parts of the ocean through photosynthe­ phosphate are quickly incorporated into the living sis, respiration, mixing, runoff, and burial

. .. - -:..~: . : TABLE 18-1 -'.-. ...• ...... • ~ .... ••• ~. " . ;-:.. ':...i:!j - • • »: Productivity and fish production of the ocean Average productivity Annual (grams of Average fish Area carbon per number of production Percentage (square square meter trophic levels (metric Area ofocean kilometers) per year) (approximate) tons)

Open ocean 90 326,000,000 50 5 160,000 Boundary-current and open­ 9.9 36,000,000 100 3 120,000,000 ocean upwelling areas" Coastal upwelling areas 0.1 360,000 300 1.5 120,000,000 Total annual fish production 240,160,000 Amount available for sustained 100,000,000 harvesting'[

Source:After Ryther, Science, 1969. • Including certain offshore areas where hydrographic features bring nutrients to the sur face. [ Not all the fish can he taken; many must be left to reproduce or the fishery will be overexploited. Oth er predators, such as seabirds, also compete with llS for the yield. 194 Marine Ecosystems and Nutrient Cycles

Or-~--'------,------'------,-----"""""

_ ~ 0.5 .- -.-_ . -I I

; ; I E I -'" ;; 1.0 ..---- -i ------I -- g. a I Atlantic

1 1.5 ····-··- -1

Pacific acifie I i 150 300 450 600 750 o 150 300 450 600 750 3 3 .. . Nitrogen as nitrate (mg/m ) Phosphorus as phosphate (mg/m )

Figure 18-9 Curves showing the vertical distribution of dissolved nitrate and phosphate in seawater from non-upwelling oceanic regions of the Atlantic, Pacific, and Indian Oceans. [Alter Gifford B. Piochot , "Marine Farming." Copyright © 1970 by Scientific American, Inc . All rights reserved.J

i \. processes. An example of a box model for phos­ ters. Similarly, respiration in the would phorus is given in Figure 18-10, where the ocean is consume all available if oxygen consump­ divided into two boxes (photic zone and deep sea) tion was not balanced by the sinking of cold, oxy­ with exchanges within and between these boxes gen-rich surface waters at high latitudes (see shown as arrows. Phosphorus enters the ocean Exercise 8). from river runoff and exits from the ocean through burial in sediments. The numerical values in parentheses show the proportion of phosphorus Nutrient Supply and Productivity cycled by each process. As the figure shows, for each atom added to the photic zone box by river Only about 10 percent of the ocean has a reason­ runoff, approximately 99 atom s are upwelled. able amount of primary productivity and signifi­ These 100 atoms are rapidly recycled in the photic cant fish production (Table 18-I). In fact, only 0.1 zone through photosynthesis and respiration percent of the ocean's environment produces about (including decomposition), but photosynthesis 50 percent of the fish available for human harvest. slightly exceeds respiration, and over approxi­ This rather startling statistic is a function of the av­ mately ten cycles a total of 100 atoms will sink out erage nutrient supply, primary productivity, and of the photic zone in the form of dead organic number of-trophic levels in each environment. As

' 1 -I matter or waste. Ninety-nine of these 100 atoms demonstrated in the table, the open ocean environ­ .: I will be respired, but one atom will be buried in ment is about 15 percent as productive as the I sediments. This system is said to be in steady state boundary-current and open-ocean upwelling en­ because for each atom lost from the system by bur­ vironments, and these are about 65 percent as ial, another replaces it through river runoff. Note productive as coastal upwelling environments. also that primary productivity would eventually However, the coastal upwelling environment pro-i cease if phosphorus removal from the photic zone duces as many metric tons of fish as do the bound­ was not balanced by upwelling of nutrient-rich wa- ary-current and open-ocean upwelling environ- Exercise 18 195 \\\\\\ Sea level

. . .: ~SynthesiS(10tl_ .

" . ~ . River runoff Plantlissue . Phosphate (1) " ' ~~."

. Upwelling' (99)

ro Q) CIl 0. Respiration Q) (99) . c

i \ Tosediments (1)

Figure 18-10 Box model of phosphorus cycling in a two-box ocean by photosynthesis, respiration, mixing, runoff, and burial processes. Numbers in parentheses represent phos­ phorus input, output, and exchange by different processes, "

rnents, while the upwelling environments as a upon by harvestable fish (Figure L8 -11b). Finally, whole produce 1500 times as much fish as does the the open-ocean region has very low nutrient remaining 90 percent of the ocean. The main rea­ replenishment and an even longer trophic web: sons for these differences are the supply of "fresh" solitary diatoms are eaten by microplankton (e.g., nutrients and number of trophic levels in each en­ radiolarians), microplankton are eaten by meso­ vironment. Coastal upwelling regions have abun­ plankton (e.g., copepods, chaetognaths), meso­ dant nutrients supplied from below and an average plankton are eaten by small, nonharvestable fish, of 1.5 heterotroph levels-their food webs are very and these small fish are eaten by harvestable fish, short and simple (Figure 18-11a). In these areas, such as tuna (Figure 18-11c), the constituents of the first trophic levelare usually Why does nutrient supply and the number of aggregates of colonial diatoms that are large trophic levels influence ultimate fish production? enough to be fed upon directly by harvestable fish. Lower nutrient supply limits primary productivity, In boundary-current or open-ocean upwelling re­ which limits total productivity. Also recall the gions, nutrients are more limited, and there are a phenomenon of ecological efficiency discussed greater number of heterotroph levels, with solitary above-only about 10 percent of the energy con­ diatoms fed upon by copepods, and copepods fed tained in a trophic level is transferred up to the 196 Marine Ecosystems and Nutrient Cycles

r, t.f Figure 18-11 Comparison of the length and makeup of food cha ins from the following "'~-1::..".,..::- areas: (a) high-productivity coastal waters; (b) a boundary-current upwelling area; (c) low­ productivity open-ocean waters. [After Gifford B. Pinchot. "Marine Farming." Copyright © 1970 by Scientific American, lnc, All rights reserved.l

. I 1 ,I next trophic level because about 90 percent is Ecological efficiency. Efficiency with which en­ used for respiration, growth, and reproduction. ergy is transferred from one trophic level to the Therefore, each intervening trophic level between next higher level. Usually expressed as a ratio or primary producers and harvestable fish incurs a percentage, it is the amount of living matter added 10-fold decrease in the biomass of harvestable fish to a trophic level compared to the amount ofliving compared to the biomass of primary producers. matter required to produce it. Thus, the longer the trophic web, the less "effi­ Heterotrophs. Organisms that obtain their food '.;-\' cient" the ecosystem is from the standpoint of har­ from other organisms. vestable fish. Nutrients. Elements, such as phosphorous and nitrogen, necessary for life processes. DEFINITIONS Trophic pyramid or web. A summary of the Autotrophs. Organisms that produce their own ways in which organisms within an ecosystem food from inorganic matter. obtain their energy, either from inorganic matter (autotrophs) or from other living creatures (het­ Decomposers. Organisms that obtain their food erotrophs). from dead organic matter. Decomposers are a par­ ticular type of heterotrophs. Report Exercise fB NAME

Marine Ecosystems DATE and Nutrient Cycles INSTRUCTOR

1. (a) Propose two physical oceanographic phenomena that could cause a poor "crop" of diatoms during a given year in the Antarctic Ocean. _

(b) How would this affect the biomass at higher trophic levels? _

2. Trophic webs can be very complex and interconnected. From Figure 18-3, determine the possible population response of the following organisms to an ofskuas. Provide reasoning for your responses. (a) Adelie penguin (one possible response) : _

(b) crabeater seal (two possible responses): --',- _

(c) leopard seal (two possible responses): _

3. Assume that the cormorant population in the Long Island estuary ecosystem (Figure 18-4) was decimated by a cormorant-specific virus . For each group of animals below, state what would initially happen to their populations. Provide reasoning for your responses. (a) flukes and eels: _

(b) water plants: _

(c) osprey and mergansers: ---.,. _

197 198 Marine Ecosystems and Nutrient Cycles

4. What would you hypothesize would eventually happen to the Long Island estuary ecosystem after the / cormorant-specific virus passed? l

5. How might you modify the Long Island estuary ecosystem to produce the following population effects? (a) decrease blowfish ; maintain fluke abundance _

(b) decrease tern abundance; maintain osprey abundance _

6. What might happen to a given ecosystem if the top carnivore biomass decreased due to disease? _

Why? _

7. Why is a more complex ecosystem (i .e., greater number of pathways) probably more stable than a simple ecosystem? ----!,. .' ~ ., ------""-,, -

8. Answer the following questions on ecological efficiency using the food web outlined for the Long Island estuary (Figure 18-4). For our purposes, assume that the water plants and marsh plants constitute the first trophic level and that the organic and plankton (copepods and diatoms) constitute the second trophic level. <, (a) Rank each bird according to its ecological efficiency. Thus. the first bird listed will obtain its food by the shortest trophic pathway (fewest levels) from the first trophic level. To calculate how directly a given species is dependent upon a particular prey item. assume that each thick line represents one biomass unit and each thin line represents one-halfbiomass unit. For example. the tern gets one unit from the silversides and a halfunit from the billfish. Some birds will share the same rating in ecological efficiency.

1. 5. _ 2. _ 6. _ 3. _ 7. _ 4. _ 8. _

(b) Assuming aU other factors (i.e.•reproduction rate, predation. etc.) equal, which bird species would you predict to be the most abundant and which the least abundant in the Long Island estuary area?

Why? ~ , ------"\, Exercise 18 199

9. The original purpose of the Long Island estuary study was focused on documenting DDT .concentration. The numbers beside each species indicate the average parts per million (ppm) of DDT found in each species. (a) What is the average amount of biological magnification of DDT from the first trophic level to the second trophic level? (As in question 8, assume that the water plants and marsh plants constitute the first trophic level.) _

(b) From the observed DDT concentrations, hypothesize which prey item the blowfish consume the most. Explain your reasoning. _

(c) Terns and mergansers are members of the fourth trophic level, but contain quite different DDT concentrations. Hypothesize two possible reasons for this difference, and state how you might test your proposed reasons. (Hint: The average age of a tern is about half that of a merganser.) _

(d) Compare your ranking of the ecological efficiency of the various birds (Question 8a) and the average concentration of DDT in their tissues. Does there appear to be any correlation? _ If so, explain the process that would produce such a pattern. _

10. In the Aleutian Islands ofthe North Pacific, Dr. J.A. Estes and co-workers at the University of California at Santa Cruz documented an abrupt decline in sea otters, which they attributed to increased predation by killer whales. While killer whales have always been a top carnivore in the open -ocean ecosystem, they appear to have recently shifted their trophic strategy to prey upon sea otters living within the shallow, nearshore ecosystem. Data from the study are provided on the following page. The basic food chain ofsea otter-sea urchin-kelp is presented on the left, and the recent addition of the killer whale to the trophic chain is shown on the right. The relative sizes of the arrows indicate the relative biomass consumption before and after the addition ofkiller whales to the ecosystem. The four graphs show available data on changes in sea otter abundance, sea urchin biomass, urchin grazing intensity, and total kelp density from 1972 to 1997. Use these data to address the following questions: (a) Describe each organism in the food chain using the trophic level terms of first-level carnivore, top carnivore, primary producer, and first-level . --- _

(b) Hypothesize some possible events that could have caused killer whales to shift their trophic strategy toward sea otters, which previously were not a major food item. What data would you need to test your hypotheses? _ 200 Marine Ecosystems and Nutrient Cycles

(

Sea otter abundance Z" 100 Q c , ::l , 8 BO •• >< • ~ 60 --- Amchitkal. C>--<:l N. Adak I. (t~~ 40 ~- ---=__ ---:::::::---- z ~ - -/:; Kagalaska L Oi 20 ...... L. Kiska I. :l:: o 0 i III III I I t t t • 1972 1985 1989 1993 1997 a Year 'I' 400 E Sea urchin biomass l/) 300 '-"! 0 200 CJ) E 0 100 0 1972 1985 1989 1993 b Year Grazing intensity I... 50 .<= ~ 40 3D ~ OOL .£ 20 ~ 0 10 0 1972 1985 1989 1993 1997 c Year (, .; ...... - / '"E Total kelp density l/) (\J 0 lii 0. zci 'iL 1972 19B5 1989 1993 1997 d Year

[From Estes, J. A., TInker, M. T., Williams, T. M., and Doak, D. F., 199B. Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science 282 :473 -476,]

(c) Describe the effects that. the addition ofkiller whales to the nearshore food chain have had on the relative biomass of organisms in each of the lower trophic levels. _

(d) In addition to the organisms shown, many others are dependent upon kelp forests for shelter, reproduction, food, etc. What may happen to this ecosystem jf the observed trend continues? _

(e) Estes et al, discuss their data and their implications with the caveat that their data are not as complete as they would like, although the general trends are clear. What additional data would you collect in the future from the nearshore or oceanic ecosystem to better understand this trophic web change? _ Exercise 18 201

11. The structure of, and interactions within, a given ecosystem can change over time scales ranging from thousands of years to hourly. Changes on hourly scales commonly occur in the intertidal zone, where the environment fundamentally changes as tides rise and fall. The general structure of salt marsh ecosystems on the Atlantic coast during high and low tide is shown below. Examine how the ecosystem structure changes through a tidal cycle and answer questions on the following page.

Low marsh at high tide

Minnow Young fish

Blue mussel

\] ; barnacle i ~) i ?ewat e r minnow ~,~ sa"d,~ml Periwinkle ~ Green crab i lsopcd t,.. Blue crab ~ "'--,

Mean low water mark

Ribbed mussel

i I . ; ~ \

Low marsh at low tide

[From Marine : An Ecological Approach, Third Edition by James W. Nybakken; Coypyright © 1993 by HarperCollins College Publishers. Reprinted by permission of Addison-Wesley Educational Publishers.) 202 Marine Ecosystems and Nutrient Cycles

(a) Which organisms are involved in the food web during both high and low tides? ---,

(b) Which organisms are the top carnivores during high tide? During low tide? _

(c) The loss ofwhich organisms would affect the structure ofthe salt marsh ecosystem during both high and low tides? _

(d) Which would be more affected by decimation ofthe periwinkle population, the high or low tide ecosystem? _

(e) Why does the ribbed mussel not show any feeding activity during low tide? _

12. In general, the rate of respiration must nearly balance that ofphotosynthesis in the ocean as a whole. However, over geologic time, there has been a slight excess ofphotosynthesis over respiration. Discuss how this fact would affect the following over geologic time: (a) composition of the atmosphere: _

(b) formation ofnatural petroleum resources: _

(c) input rate of nutrients to the ocean versus their burial rate: _