"Exercise 18

Marine Ecosystems andNutrient Cycles

-x,

- - .:.~ ~" .

. ... .. ,1

:-,:- :. -:~ :

. ~ :.: :

- '"'"'-- j - -_:-.~. ~.~ .:..

':~

of energy may be represented by a trophic pyra­mid (Figure 18-2) or trophic web (Figures 18-3and 18-4). The second property is that individualnutrients, elements necessary for metabolicprocesses, are recycled many times within mostecosystems, with decomposers playing a crucialrole in the release of nutrients from organic matter.

Trophic Pyramids and Webs:Examples from the AntarcticOceanA simplified trophic pyramid for the AntarcticOcean is presented in Figure 18-2. Diatoms are theprimary producers, providing energy for the entireecosystem, and are shown at the base of the pyra­mid. These primary producers are consumed bythe primary conswners (herbivores) of the trophicpyramid's s~cond tier, mostly krill. In turn, krill arethe energy source for the third trophic tier, thewhales. The whales are termed secondary con- , ;.~~ '~,",'

::~~~,~h(;h~~'~::'~~~~=~:~s~~ first-level ~f~~ _-~In Figure 18-3, a more realistic model of en­

ergy .flow through the Antarctic Ocean ecosystem ispresented. This trophic web is essentially a trophicpyramid expanded to include the more complexinterrelationships between organisms at highertrophic levels; the trophic relationship between thediatoms (primary producers) and krill (secondaryconsumers or herbivores) rema.ins the same. Notethat there is a greater diversity of organisms at thehigher trophic levels and that some of these canoperate at multiple trophic levels. In this trophicweb, blue whales remain in the third trophic level,

OBJECTIVES:

1/1 To understand the role of the ocean as an ecosys­tem and nutrient recycler.

II To understand the interactions between and flowof energy through producers, consumers, and de­composers.

II To appreciate how hwnans can disrupt marineecosystems.

';Ill this exercise we explore how energy from pri­mary productivity (Exercise 16) is transferredthrough an ecosystem. An ecosystem consists of

a group of Jiving organisms, the physical environ­ment in which they live, and an energy source (e.g.,sunlight .in photosynthesis-based ecosystems). Thelargest ecosystem can be considered the earth as awhole; the planet may be subdivided into terres­trial and marine ecosystems, and each of these maybe further subdivided, often on the basis of envi­ronmental conditions (e.g., depth, temperature,etc.) , In each ecosystem, there are organisms thatproduce food (primary producers or autotrophs),organisms that consume other organisms (sec­ondary producers or heterotrophs), and organ­isms that decompose autotroph and heterotrophwaste products and bodies after death (decom­posers; generally fungi and bacteria). Ecosystemshave two fundamental properties (Figure 18-1).The first is that energy flows through an ecosystemin only one direction: it is received from the sun,transformed into organically usable forms throughprimary producers, and flows to secondary pro­.ducers and decomposers. Depending upon the:'.,:omplexity of the ecosystem, this one-way transfer

, - , ~..~~-.

.. --";-,' :- ~' ,.

, _- I

.; '- "I, I

- -"

~~~l~~

,· ' 0"

f~~'

. ~:.

: ' ~ ,.

.,.. - .- :

" -~ -,. "

.:~-.: ~.'

185-~~ ""-t ""<

".-. ~-: ~: ... :::- "".s::::".....

.::::~ ..:.~.~:

-+---------

186 Marine Ecosystems and Nutrient Cycles

Primaryproducers

iI

II

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 krillare also included (i.e., crabeater seals, wingedbirds, Adelie penguins, and small fish and squid) .The small fish and squid, in turn, are prey items foremperor penguins, larger fish, and Weddell andRoss seals-members of the fourth trophic level.Because skuas feed upon the chicks of Adelie pen­guins, they are also considered members of thefourth trophic level. The remaining organisms inthe trophic web, the leopard seals and killer whales,occupy multiple trophic levels, as they feed uponmultiple trophic levels below them. For example,leopard seals may be assigned to the fourth trophiclevel when feeding upon crabeater seals or Adeliepenguins, or the fifth trophic level when feedingupon emperor penguins, large fish, or Weddell andRoss seals. Similarly, the killer whale belongs to thefourth trophic level when feeding upon bluewhales or crabeater seals, or to the fifth trophiclevel when feeding upon leopard seals. Note thatsome organisms may shift their trophic positionduring their lifetime; for example, as fish growlarger, some shift from the second to third trophiclevel. Appreciate that 13 different species are di­rectly dependent upon krill, which are themselvesdependent upon diatoms.

Compare the Antarctic food web to that of th(, .;.,~:~:.~.; j

Long Island estuary (Figure 18-4). Notice thattrophic levels in the estuary increase from left toright, from primary producers (plants, phyto­plankton) to top carnivores (birds). Such complex­ity is characteristic of many marine food webs,with many interrelations between organisms.Ecological theory states that the greater the num­ber of food pathways leading from the primaryproducers to higher trophic levels, the more resis­tant the ecosystem will be to disturbances relatedto losses of individual species . Why might thisbe so?

Ecological Efficiency and BiologicalMagnification

Energy contained within an ecosystem is not recy­cled, but moves unidirectionally through succes­sively higher trophic levels. Within a given trophiclevel, the vast majority of energy obtained from thelevel below is used for respiration and metabolismand is lost as excretion or heat; only a small po~ .: • .tion is transformed into biomass through growth. .In addition, not all available individuals in the

· 1., I': 1

Exercise 18

lower trophic level will be consumed; some will dieof natural causes. Both of these factors result in aphenomenon termed ecological efficiency, whichexpresses the amount of energy (biomass) flowinginto a trophic level compared to the amount of en­ergy retained within that trophic level. In mostecosystems, ecological efficiency is less than 10 per­cent. For example, in Figure 18-2, the numbers onthe left indicate that of the roughly 1000 biomassunits of the first trophic level, only 100 units (- 10percent) of this energy is converted into biomass atthe second trophic level. Another way to visualizethis relationship is that 100 grams of diatoms arerequired to support 10 grams of krill, and 10grams of krill are required to support I gram ofblue whale. From a fishery standpoint, the greaterthe number of trophic levels between primary pro­ducers and edible fish, the less efficient the ecosys-

187

tem is in converting solar energy into food ,prod­ucts for humans.

Related to this concept of ecological efficiencyis the biological concentration of pesticides, suchas DDT, in the marine ecosystem. DDT is an ex­tremely effective agent against agriculture-damag­ing insects. and has been in extensive use since the1940s, although banned in the United States since1972. One reason the pesticide is so effective is itshigh resistance to biological breakdown. This resis­tance leads to DDT's eventual transport from agri­cultural fields through irrigation and runoff intothe ocean, where it is incorporated into the marinebiosphere during primary production. Like mostpesticides, DDT is damaging to non-insects as well;in phytoplankton, it decreases the efficiency ofphotosynthesis and therefore reduces the total pri­mary 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

~..-

Weddell seal

Blue Whale

Leopard seal

....---------------- ~,I,

IIIIIIIII: ...~--------------~I'

(I,,,

... ... ...

.... Krill

------ . ~~--.:::::::::::::::lE".,c~!L..-f;,,;~~~~. f. "'~:' ';l.::tl:.~ i ;.;.~

t . ~...... ... ...

......,,

\\,II

)~"r

~----------------------------~~:/;" ~ ..."7' W' d bi d I.; »" mge rr s ,I" / Adelie penqu ln ~ ,,1(" :" ...--------~Q ~ ~-_-_-_-_-_- .;/f

I " .... -3"' . . II, /" ., ..' Small fish I

I' " squid ."~~. -54 Empe'o' penguin _ -~_--

~·-----------------::,,1Large fish / :lJ<_~ ...':

I,IIII

,,/ III,

IRoss seal /

,~------>~~-----------_ ....:,I

,I ft., I... I

...... I... I" ,...... ...

.... :-----2":::=._-----

Figure 18-3 Summary of major t rophic relationships with in the Ant arctic

ecosystem. [After Robert C. Murphy, "The Oceanic Life of the Antarct ic ."

Copyright © 1962 by Scient ific American, Inc. All rights reserved.]

Exercise 18 189

Greenheron3.57,3.51

.~~n 3.15, 5.17, 4~75, 6.40

~.~~ 1.....r...Y -;J , L~..~, . ....

Billfish 2.07 I

Osprey (egg) 13.8

Minnow 1.24

Marsh plantsf/!l: Shoots 0.33

~~Z C!!lilri:.*---------.---+- ENERGY FLOW

Water plants0.08

Organic debrisMarsh 13 pounds per acreBottom 0.3 pound per acre

fi

II

II

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. WoodweII, "Toxic Substances and Ecological

Cycles." Copyright © 1967 by Scientific American, Inc. All right s reserved.l

Unfortunately, the effects of DDT on the marineecosystem do not end at the first trophic level.Because it is not easily broken down, the pesticideis stored in body tissue and transferred up throughhigher trophic levels. When DDT-enriched phyto­plankton are consumed by primary consumers, the10 percent efficiency effect applies to organic com­pounds, but not to DDT. Thus, the DDT is pas­sively transferred through the ecosystem. Whilesome DDT is given off through respiration and ex­cretion at each level, the bulk remains in the bio-

mass and becomes increasingly concentrated athigher trophic levels, becoming the highest in thetop carnivores (Figure 18-5).

This phenomenon is called biological magnifi­cation, and the resulting DDT concentrations canwreak havoc with biological processes rangingfrom eggshell calcification in birds to reproductionin humans. For example, in the final years beforethe 1972 U.S. ban on DDT, brown pelicans, whichnest on islands off the coast of southern California,had acquired so much DDT that most of their

190 Marine Ecosystems and Nutrient Cycles

Carbon Cycle. Carbon is the basic buildingblock of all organic molecules. Its biogeochemicalcycle is presented in Figure 18-6. Note that carbonis rarely limiting in most marine ecosystems; onlyabout 1 percent of the carbon reservoir in theocean-is involved in primary productivity at anygiven time, Another major carbon reservoir is at­mospheric carbon dioxide (C02) , which enters theocean by gas exchange at the air-water interface.Additional carbon dioxide comes from the respira­tion of plants and animals. Thus, dissolved carbondioxide is readily available to autotrophs for pho­tosynthesis, which «fixes" carbon into organic mol­ecules that are then transferred through thetrophic web by predation and feeding. Carbon isalso present in carbonate sediments and limestonescomposed of the calcareous skeletons of marineorganisms, and as natural petroleum deposits (i.e.,oil, gas, peat, coal) formed by burial of ancient or­ganic material. Carbon within these reservoirstends to have a long residence time (the averagetime interval that individual particles remain in a.given reservoir), but is ultimately cycled through'terrestrial weathering in dissolved form to the

Biogeochemical Cycles

The main factor limiting productivity in well-litmarine waters is the availability of principal nutri­ents such as nitrate, phosphate, and carbon (al­though carbon is not limiting in most regions).Here we will examine the cycling of these nutrientsthrough various "reservoirs" in the atmosphere,lithosphere, hydrosphere, and biosphere, and seehow their distribution and exchange affects themarine ecosystem, The general biogeochemical cy­cle involves the intake of an inorganic form of anutrient by an autotroph during photosynthesis oforganic molecules, which are subsequently trans­ferred through the trophic web by heterotrophicactivity. Eventually; decomposers transform deadorganic matter and waste back into its inorganicform, making the constituent nutrients availableagain to autotrophs. Keep in mind that decom­posers, such as bacteria and fungi, are critical com-:ponents in recycling such nutrients. '.

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. .

. . , DDT

(

Losseslhroughrespirationexcretion

o Biomass

"

, ,

..

. ,

',.,

· "

· ".. :'.

".· .

Plant

Carnivore 1

Carnivore2

eggshells would break when incubated, causing thepelican population to drop precipitately. Since the1972 ban, the pelican population has recovered,but DDT concentrations remain relatively high.Concentrated levels of DDT have also been foundin Adelie penguins and skuas, which indicates thatthe pesticide has been spread through oceanic cir­culation and other processes to regions far re­moved from its original application. You most

Figure 18-5 Biological magnification of DDT in a ma­

rine food chain . [After George M, Woodwell, "Toxic

Substances and Ecological Cycles," Copyright © 1967 by

Scientific American, Inc. All rights reserved.l

)H'~

.

~. " ....

J

:'.

Exercise 18 191

" ~

i

II

Figure 18-6 The biogeochemical cycle of carbon in organic and inorganic forms .

[After Harold V. Thurman, "Essentials of Oceanography." Copyright © 1983 by Charles E.

Merrill Publishing Co. All rights reserved .]

- Organic nitrogencompounds

_Inorganic nitrogencompounds

Denitrifying d7l~ "",

bacteria bacteria -f'J~

~:,:-¢t?J~~ Bacteria~ Ammonium

nitrogen --------: J.~\" .~

~, Nitrite -Nitrous

~acteria oxides

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-relatedburning into the atmosphere.

Nitrogen Cycle. Nitrogen is a essential elementin the production of amino acids-the building

blocks of proteins within all living things. The bio­geochemical cycle of nitrogen is summarized inFigure 18-7 and involves a fairly comple: suite ofbacterial "fixers" and decomposers. Molecular ni­trogen (N 2) cannot be used directly by most

192 Marine Ecosystems and Nutrient Cycles

organisms, but metabolic processes within cyanobac­teria convert dissolved nitrogen into nitrate (NO j )

through a process called nitrogen fixation. The re­sulting nitrate is the nutrient form most easily uti­lized by most phytoplankton and is subsequentlytransferred up the trophic web through feeding.

Excreted waste and dead organic matter arebroken down by decomposers as an energy source.Some of these decomposers are denitrifying bac­teria, whose metabolism breaks down organic­bound nitrogen into progressively oxidized inor­ganic forms: the first is ammonia (NH3) , followedby nitrite (N02) , or finally nitrate (N03) . The lessoxidized forms (ammonia, nitrite) are generallytaken up again by autotrophs before denitrifyingbacteria can fully oxidize the compounds to ni­trate. Only a small percentage of nitrogen initiallyfixed by cyanobacteria is recycled within the photiczone; the majority is not oxidized back into a us­able form until it is well below the photic zone andthus unavailable to photosynthesizing autorrophs,The major mechanisms for returning this fixed ni­trogen to the photic zone are seasonal vertical mix­ing of the water column and local upwelling of

nutrient-rich bottom waters. Some denitrifying..--...bacteria will completely oxidize the nitrogen-bear/ . 'ing compounds back into molecular nitrogen,"some of which may be exchanged with the atmos­phere. In addition, small amounts of nitrogen areburied in ocean sediments and released throughterrestrial weathering-for simplicity and becauseof their relatively small contribution to the marinecycle, these paths are not shown in Figure 18-7.

Phosphorus Cycle. Phosphorus is an essentialelement in all living organisms' genetic informa­tion (DNA and RNA) and in the ATP compoundsinvolved in the conversion of carbohydrates intoenergy. Its biogeochemical cycle is simpler thanthat of nitrogen, largely because the bacterial com­ponent of the cycle is simpler (Figure 18-8).Phosphorus is released to the ocean through theweathering of phosphate-bearing rock and re­moved through burial of organic matter (notshown). Inorganic orthophosphate may be useddirectly by autotrophs and, like nitrogen, is trans­ferred to higher trophic levels through feeding. Thephosphorus within excreted waste and dead or-

i

~, ' ' :

--" .".,

Phosphates freed byweathering and erosion

Guano andbone remains

..:

Mechanical and autolytic/"release of phosphates~

Dead plant

tissue ~

?~~. .'~ : . :...J . ~sue

~ Deadf'~ \Deaq, plantanlm~1 tissue

tissue UsableI , phosphates

\~~LJJ

- Organic phosphoruscompounds

_ Inorganic phosphoruscompounds

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

ganic matter is released back into the environmentthrough several pathways-all much faster thanthose for nitrogen. As a result, phosphorus is a lesslimiting nutrient in the photic zone, even thoughits concentration is roughly one-seventh that of ni­trogen. As with nitrogen, some phosphorus may bedepos ited in ocean sediments or in coastal regionsin the form of bird guano; these materials areeventually recycled through weathering and ero­sion back into the ocean.

Nutrient Distribution in the OceanThe availability of nitrogen and phosphorus oftenlimits primary productivity and the resultingphotic zone biomass. As shown in Table 18-1, theopen ocean constitutes roughly 90 percent of thetotal oceanic environment and is often considereda "biological desert" because of the paucity of dis­solved nutrients within the photic zone. Typicalvertical concentrations of dissolved nitrate andphosphate for different oceans are shown inFigures 18-9. These nutrient patterns are the direct

. result of the biogeochemical cycling discussedabove. Dissolved nutrient concentrations are lowin the photic zone because available nitrogen andphosphate are quickly incorporated into the living

193

biomass through primary productivity. In addi­tion, significant amounts of nutrients are exportedfrom the photic zone through the continuous or­ganic "rain" of fecal material, carcasses, and molts.while bacterial decomposition of this organic"rain" continues beneath the photic zone, the re­leased nutrients are effectively sequestered fromthe photic zone by the strong density differencebetween the mixed layer and the deep-water layer,as marked by the thermocline. Thus, if vigorousproductivity is to take place, these nutrient-richdeep waters must somehow be transported up­ward to "fertilize" the nutrient-poor photic zone.Physical mechanisms that accomplish this fertiliza­tion include upwelling and the weakening of thethermocline during the winter season or intensestorms. Note that the absolute concentrations ofdissolved nitrogen and phosphorus are higher inthe deep waters of the Pacific and Indian oceansthan in the Atlantic. This is because bottom watersin the Pacific and Indian oceans have been in thethermohaline circulation system for a longer timeand have thereby accumulated more dissolved nu­trients from overlying surface waters.

Oceanographers often construct "box models"to summarize how nutrients are cycled throughdifferent parts of the ocean through photosynthe­sis, respiration, mixing, runoff, and burial

TABLE. .. - -:..~: . :

18-1 -'. -. ...•. ...... • ~ .... •• • ~. " . ;-:.. ':...i:!j - • • »:

Productivity and fish production of the ocean

Area

Open oceanBoundary-current and open­

ocean upwelling areas"

Coastal upwelling areas

Total annual fish production

Amount available for sustainedharvesting'[

Averageproductivity Annual

(grams of Average fishArea carbon per number of production

Percentage (square square meter trophic levels (metricof ocean kilometers) per year) (approximate) tons)

90 326,000,000 50 5 160,000

9.9 36,000,000 100 3 120,000,000

0.1 360,000 300 1.5 120,000,000

240,160,000

100,000,000

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 competewith llS for the yield.

194 Marine Ecosystems and Nutrient Cycles

750

acifie

I-I;

IAtlant ic

1

····-··- -1

o 150 300 450 600Phosphorus as phosphate (mg/m3)

750

PacificI

300 450 600Nitrogen as nitrate (mg/m3)

-------- I --

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

i150

0.5 .- -.-_ _~ .

;II

1.5

E-'";; 1.0 ..--- - -i

g.a

.. .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\.

' 1-I

.: II

processes. An example of a box model for phos­phorus is given in Figure 18-10, where the ocean isdivided into two boxes (photic zone and deep sea)with exchanges within and between these boxesshown as arrows. Phosphorus enters the oceanfrom river runoff and exits from the ocean throughburial in sediments. The numerical values inparentheses show the proportion of phosphoruscycled by each process. As the figure shows, foreach atom added to the photic zone box by riverrunoff, approximately 99 atom s are upwelled.These 100 atoms are rapidly recycled in the photiczone through photosynthesis and respiration(including decomposition), but photosynthesisslightly exceeds respiration, and over approxi­mately ten cycles a total of 100 atoms will sink outof the photic zone in the form of dead organicmatter or waste. Ninety-nine of these 100 atomswill be respired, but one atom will be buried insediments. This system is said to be in steady statebecause for each atom lost from the system by bur­ial, another replaces it through river runoff. Notealso that primary productivity would eventuallycease if phosphorus removal from the photic zonewas not balanced by upwelling of nutrient-rich wa-

ters. Similarly, respiration in the deep sea wouldconsume all available oxygen if oxygen consump­tion was not balanced by the sinking of cold, oxy­gen-rich surface waters at high latitudes (seeExercise 8).

Nutrient Supply and ProductivityOnly about 10 percent of the ocean has a reason­able amount of primary productivity and signifi­cant fish production (Table 18-I) . In fact, only 0.1percent of the ocean's environment produces about50 percent of the fish available for human harvest.This rather startling statistic is a function of the av­erage nutrient supply, primary productivity, andnumber of-trophic levels in each environment. Asdemonstrated in the table, the open ocean environ­ment is about 15 percent as productive as theboundary-current and open-ocean upwelling en­vironments, and these are about 65 percent asproductive as coastal upwelling environments.However, the coastal upwelling environment pro-iduces as many metric tons of fish as do the bound­ary-current and open-ocean upwelling environ-

Exercise 18

\\\\\\Sea level

195

i

\

roQ)CIl0.Q)<Ilc

. . .: ~SynthesiS(10tl_ ." . ~ .

Plant lissue . Phosphate

" ' ~~."

. Upwelling'(99)

Respiration(99) .

Tosediments(1)

River runoff(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 awhole produce 1500 times as much fish as does theremaining 90 percent of the ocean. The main rea­sons for these differences are the supply of "fresh"nutrients and number of trophic levels in each en­vironment. Coastal upwelling regions have abun­dant nutrients supplied from below and an averageof 1.5 heterotroph levels-their food webs are veryshort and simple (Figure 18-11a). In these areas,the constituents of the first trophic levelare usuallyaggregates of colonial diatoms that are largeenough to be fed upon directly by harvestable fish.In boundary-current or open-ocean upwelling re­gions, nutrients are more limited, and there are agreater number of heterotroph levels, with solitarydiatoms fed upon by copepods, and copepods fed

upon by harvestable fish (Figure L8-11b). Finally,the open-ocean region has very low nutrientreplenishment and an even longer trophic web:solitary diatoms are eaten by microplankton (e.g.,radiolarians), microplankton are eaten by meso­plankton (e.g., copepods, chaetognaths), meso­plankton are eaten by small, nonharvestable fish,and these small fish are eaten by harvestable fish,such as tuna (Figure 18-11c),

Why does nutrient supply and the number oftrophic levels influence ultimate fish production?Lower nutrient supply limits primary productivity,which limits total productivity. Also recall thephenomenon of ecological efficiency discussedabove-only about 10 percent of the energy con­tained in a trophic level is transferred up to the

196 Marine Ecosystems and Nutrient Cycles

Figure 18-11 Comparison of the length and makeup of food cha ins from the following

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

r,

t.f"'~-1::..".,..::-

. I1,I

'.;-\'

next trophic level because about 90 percent isused for respiration, growth, and reproduction.Therefore, each intervening trophic level betweenprimary producers and harvestable fish incurs a10-fold decrease in the biomass of harvestable fishcompared to the biomass of primary producers.Thus, the longer the trophic web, the less "effi­cient" the ecosystem is from the standpoint of har­vestable fish.

DEFINITIONS

Autotrophs. Organisms that produce their ownfood from inorganic matter.

Decomposers. Organisms that obtain their foodfrom dead organic matter. Decomposers are a par­ticular type of heterotrophs.

Ecological efficiency. Efficiency with which en­ergy is transferred from one trophic level to thenext higher level. Usually expressed as a ratio orpercentage, it is the amount of living matter addedto a trophic level compared to the amount of livingmatter required to produce it.

Heterotrophs. Organisms that obtain their foodfrom other organisms.

Nutrients. Elements, such as phosphorous andnitrogen, necessary for life processes.

Trophic pyramid or web. A summary of theways in which organisms within an ecosystemobtain their energy, either from inorganic matter(autotrophs) or from other living creatures (het­erotrophs) .

Exercise fBNAME

Report

Marine Ecosystemsand Nutrient Cycles

DATE

INSTRUCTOR

1. (a) Propose two physical oceanographic phenomena that could cause a poor "crop" of diatoms during agiven 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 possiblepopulation response of the following organisms to an overpopulation of skuas. Provide reasoning for yourresponses.

(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) wasdecimated by a cormorant-specific virus . For each group of animals below, state what would initiallyhappen 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 abundance; 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 asimple ecosystem? ----!,. .'

~ .,

--------------- - - - - - - - ------- - - - - - - - - - - ""-,, -

8. Answer the following questions on ecological efficiency using the food web outlined for the Long Islandestuary (Figure 18-4). For our purposes, assume that the water plants and marsh plants constitute the firsttrophic level and that the organic detritus and plankton (copepods and diatoms) constitute the secondtrophic level. <,

(a) Rank each bird according to its ecological efficiency. Thus. the first bird listed will obtain its food bythe shortest trophic pathway (fewest levels) from the first trophic level. To calculate how directly a givenspecies is dependent upon a particular prey item. assume that each thick line represents one biomassunit and each thin line represents one-half biomass unit. For example. the tern gets one unit from thesilversides and a half unit from the billfish. Some birds will share the same rating in ecological efficiency.

1.2. _3. _4. _

5. _6. _7. _8. _

(b) Assuming aU other factors (i.e.• reproduction rate, predation. etc.) equal, which bird species wouldyou 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 DDTfound in each species.

(a) What is the average amount of biological magnification of DDT from the first trophic level to thesecond trophic level? (As in question 8, assume that the water plants and marsh plants constitute thefirst trophic level.) _

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

(c) Terns and mergansers are members of the fourth trophic level, but contain quite different DDTconcentrations. Hypothesize two possible reasons for this difference, and state how you might test yourproposed 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 averageconcentration 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 of the North Pacific, Dr. J.A. Estes and co-workers at the University ofCalifornia at Santa Cruz documented an abrupt decline in sea otters, which they attributed to increasedpredation by killer whales. While killer whales have always been a top carnivore in the open -oceanecosystem, they appear to have recently shifted their trophic strategy to prey upon sea otters living withinthe shallow, nearshore ecosystem. Data from the study are provided on the following page. The basic foodchain of sea otter-sea urchin-kelp is presented on the left, and the recent addition of the killer whale to thetrophic chain is shown on the right. The relative sizes of the arrows indicate the relative biomassconsumption before and after the addition of killer whales to the ecosystem. The four graphs showavailable data on changes in sea otter abundance, sea urchin biomass, urchin grazing intensity, and totalkelp 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, topcarnivore, primary producer, and first-level herbivore. --- _

(b) Hypothesize some possible events that could have caused killer whales to shift their trophic strategytoward sea otters, which previously were not a major food item. What data would you need to test yourhypotheses? _

200 Marine Ecosystems and Nutrient Cycles

(, .;.... . .- /

(

1997t t •I I t

1993I I I

1989 1993 1997

Year

Total kelp density

1989 1993 1997

Year

1989Year

Sea urchin biomass

1985 1989 1993Year

Grazing intensity

Sea otter abundanceQ,,

•••--- Amchitkal.C>--<:l N. Adak I.

~ - -/:; Kagalaska L

.............. L. Kiska I.

a

'I' 400E 300l/)

'-"!0 200CJ)

E 10000

1972b

OOLI... 50.<=

~40

~3D

.£ 20~ 100

01972 1985

c

'"

'iLEl/)(\J

0lii0.

ciz1972 19B5

d

Z" 100c::l

8 BO><~ 60

(t~~ 40~- ---=__ ---:::::::---- z

Oi 20:l::o 0 i I I I

1972 1985

[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 of killer whales to the nearshore food chain have had on therelative 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 ascomplete as they would like, although the general trends are clear. What additional data would youcollect in the future from the nearshore or oceanic ecosystem to better understand this trophic webchange? _

Exercise 18 201

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

Low marsh at high tide

MinnowYoung fish

sa"d,~ml

Green crab~ i lsopcd

Blue crab ~

Blue musselbarnacle

~)

~i?ewater

minnow

,~Periwinkle

t,.."'--,

\] ;

i

i

I

Mean lowwater mark

Ribbedmussel

. ;~\

Low marsh at low tide

[From Marine Biology: 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 of which organisms would affect the structure of the salt marsh ecosystem during both highand low tides? _

(d) Which would be more affected by decimation of the periwinkle population, the high or low tideecosystem? _

(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 of photosynthesis in the ocean as a whole.However, over geologic time, there has been a slight excess of photosynthesis over respiration. Discuss howthis fact would affect the following over geologic time:

(a) composition of the atmosphere: _

(b) formation of natural petroleum resources: _

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