Ocea 130/230 Spring 2005Biological OceanographyTrophic Structure and Food WebsReading: Miller, pages 74-78, Chapter 5Optional Reading: Valiela, Chapter 9Optional Reading: Lalli and Parsons, pp. 112-136I. Modeling Trophic Dynamics• 1946, Riley predicted that you could determine the standing stock (“crop”) ofphytoplankton based on a combination of other factors. He used a multiple-regressionmodel to determine that:PP = 153t – 120P –7.3N –9.1Z + 6713Where PP = phytoplankton biomasst = temperatureP, N = phosphorous, nitrogenZ = Zooplankton• 1947, He simplified this to a general mathematical model that stated:dN/dt = N(Ph –R) – GWhere dN/dt = change in population of phytoplanktonPh = phytoplankton growth rateR = respirationG = grazing• These models account for several factors:• “Bottom Up” Controls, e.g. Temperature, Nutrients, Light• “Top Down” controls, e.g. being eaten by someone else• “Trophic Cascades”, e.g. if you increase nutrients, you increase phytoplankton,which causes an increase in zooplankton, and (eventually) an increases in fishes• Within the model, all of the important factors, such as light and nutrient parameters,species dependency, etc. are accounted for by the sub-equations—Riley went on toexplicitly include Michaelis-Menten kinetics, uptake versus irradiance, temperaturedependence, respiration by grazers, etc.• The Riley model can be combined with the concept of r versus K strategies:- We can divide all species based on their reproductive strategy- r strategists are opportunistic species, characterized by:- variable climate- small sizeOcea 130/230 Spring 2005Biological Oceanography- fast growth- not very competitive- K strategists are equilibrium species, characterized by:- constant climate- large size- slow growth- very competitive• When combined with the size/density concept (review the Chisholm paper), we comeup with the idea of a “food chain”II. Food Chains• Based on the Riley model, the classic food chain describes the transfer of energy fromone group to the next• Each level is a trophic group (or level), starting with primary producers—next levelwould be secondary consumers (herbivores), then tertiary, etc. ending with an apexpredator• An important thing to remember is that the food chain is actually describing thetransfer of energy as organic material, and it only goes in one direction• With each step, following the r-K dichotomy, we expect size to increase, lifespan toincrease, and abundance to decrease (the food pyramid idea)• If we assume that the chain is really linear, we can predict a Transfer Efficiency:ET = Pt / ( t-1)Where E is the efficiency, P is the annual production, and t, t-1 are the trophiclevels we’re looking at- In general, we assume about a 10% efficiency from one step to the next,meaning that 10% of the energy (or material) is transferred, 90% goes back tothe environment (Sunlight to plant efficiency is on the order of 1%, plant toherbivore is on the order of 20%, every other step is about 5-20%)- This also explains the biological pump, which transfers about 1-10% of annualprimary production to depth• This also implies that we can model the availability, abundance, etc. of organismsbased on our knowledge of the community production…for example, we should beable to predict a stable fisheriesIII. Food Webs• The preceding idea assumes that energy moves more or less linearly, and that thereare relatively few organisms involved…the classic example from around here is that:Physics -- > Diatoms -- > Krill -- > Blue WhalesOcea 130/230 Spring 2005Biological Oceanography• Things are rarely that simple, and you will usually be wrong if you try to model bluewhale abundance (for example) based on the physics of California• In the last 25 years, there’s been a paradigm shift from the linear, food chain model tothe complex (and complicated) food web model. This was caused by a combination offactors:1) New methods of measurement discovered new groups and abundances oforganisms in the ocean2) We discovered that small organisms are very important!3) Microbial processes (rates) are equally important4) Bacterial abundance is much higher than previously thought5) The collapse of the major fisheries called into question our ability tomathematically model the transfer of energy• This paradigm shift was initiated in 1977, when Hobbie published his method ondirectly counting bacterial cells in the ocean using Acridine Orange• As a side note, we don’t usually use Acridine Orange any more, because it isfairly indiscriminate in labeling cells (dead or alive). The preferred stain isnow DAPI, which only stains double-stranded DNA• 1980’s, Chisholm and Olson “discovered” prochlorophytes• 1983, Azam coined the term “microbial food loop” to refer to the recycling of organicmaterial independent of the classic food chain• 1990’s, Azam, Fuhrman, and others discovered the importance of marine viruses• 2000’s, the role of bacteria and viruses becomes increasingly importantBased on these findings, we now need to add several steps in the food chain:1) Viruses• 0.02 – 0.2 microns in diameter• Approximately 10E6-10E9 per mL• they are species-specific, meaning they have a particular host organism,and won’t attack species indiscriminately• Can account for 25-100% of phytoplankton mortality• Up to 50% of the phytoplankton might contain viruses at any given time2) Bacteria• 0.2 – 1.0 microns• 10E5-10E8 per mL• On average, about 40-60% of primary production goes to bacterialrespiration• They can use Dissolved Organic Matter (DOM) directly, attack othercells, or use inorganic nutrients• They can be extremely motileOcea 130/230 Spring 2005Biological OceanographyDissolved Organic Matter: this is functionally defined as anything that’s organic (has acarbon chain) that passes through a filter, typically a 0.2 µm filter. Sometimes a GF/Fglass fiber filter is used, which has a pore size of about 0.7 µm. DOM is made up of(typically):DOC (dissolved organic carbon), including carbohydrates, lipids, proteins, etc.DOP (dissolved organic phosphorous), including ATP, ADP, RNA, DNA, etc.DON (dissolved organic nitrogen), including urea, amino acids, etc.We don’t usually care what, exactly, the compounds are, just that they are dissolved.Some of it is very long-lived (not easily used by organisms for