Transcript Lecture 6x
Lecture 6: Plankton Plankton: Definitions • Plankton: organisms living in the water column, too small to be able to swim counter to typical ocean currents – Holoplankton – spend entire life in water column – Meroplankton – spend part of life in water column, are benthic for remainder of life Plankton: Definitions • Phytoplankton – photosynthetic protists and bacteria. Single celled or chains of cells. • Zooplankton – nonphotosynthetic protists and animals. Range from single celled to small vertebrates. • Mixoplankton (or mixotrophic) - can be classified at several different trophic levels Plankton Size Classes Position in Water Column • Phytoplankton must be near a source of sunlight – 50-100 m in open ocean – Shallower depths in inshore waters and estuaries • Zooplankton usually feed on phytoplankton, or organisms that feed on phytoplankton Vertical Position in Water Column • Ways to avoid sinking (neutral buoyancy): – Regulate bulk density (the mass of an organism divided by its total volume) by varying chemical composition – Gas secretion – Body shape – Swim Vertical Position of Plankton • Smaller organisms denser than seawater sink with a constant velocity, proportional to organismal mass (Stokes’s Law) • Heavier organisms will sink faster than lighter organisms • Irregularly shaped plankton sink slower than predicted by Stokes’s Law Phytoplankton • Numerous groups, including many flagellated types • High diversity • Different groups have different nutrient needs (e.g., Fe, Si, Ca, P, N) • Different groups have different properties such as bulk density, ability to swim Phytoplankton • Plantlike Single-celled Protists – – – – – – Diatoms Dinoflagellates Coccolithophores Silicoflagellates Green algae Cryptomonad flagellates • Cyanobacteria Zooplankton • Crustacean Zooplankton • Other – – – – Copepods Krill Cladocera Others • Gelatinous Zooplankton – – – – Cnidarians Ctenophores Salps Larvacea – Arrow Worms – Pteropods – Planktonic polychaetes • Animal-like Protists – Ciliates – Foraminifera – Radiolaria Zooplankton Critical Factors in Plankton Abundance Major Physical Factors Affecting Primary Production • • • • Temperature Light Hydrodynamics Nutrients Patchiness of the Plankton & Its Causes • Spatial changes in physical conditions behavioral responses and population growth/mortality responses • Water turbulence and current transport • Spatially discontinuous levels of grazing • Localized reproduction • Social behavior Wind and Turbulence • Wind can affect patchiness at a wide range of spatial scales – Langmuir circulation – wind driven water movement creates small vortices which result in small divergences and convergences of water • Result in linear convergences at surface Directional Flow and Obstructions • Directional water flow can cause persistent spatial patterns in circulation • Flow patterns can be altered at obstructions (islands, mouths of estuaries, passes, etc. ) Depth and Plankton Layers • Phytoplankton and small zooplankton can be concentrated in layers at different water depths Patchiness of the Plankton • Concentrated patch of phytoplankton must eventually disperse due to the transfer of wind and current energy into kinetic energy Phytoplankton Patchiness • Population density determined by interaction between turbulence and population growth • Blooms probably caused when you have a rapid increase in phytoplankton growth in an area with restricted circulation Spring Phytoplankton Bloom • Predictable seasonal pattern of phytoplankton abundance in the temperate and boreal waters of depths of ~10-100m • Spring diatom increase = phytoplankton increase dramatically and are dominated by a few diatom species Phytoplankton, Zooplankton, Nutrients, and Light Throughout the Year in Temperate-Boreal Inshore Waters Latitudinal Variation in Cycle Geographical Comparisons of Primary Production Polar Seas Temperate Seas Tropical Seas Light Well lit in summer Light varies seasonally Well lit throughout year Stratification No stratification Seasonal stratification Occurs throughout year Nutrients Unlimited Mixing replenishes nutrients Low nutrient content in surface waters Primary Production Only occurs in icefree summer but can be quite substantial Major peak in spring with minor peak in fall Low but constant year-round Spring: small, rapidly growing diatoms; Summer: larger diatoms; Late Summer/Fall: dinoflagellates; Winter: Small diatoms Dinoflagellates dominate year-round Successional Patterns No real succession because production only occurs in summer Light and Phytoplankton • Light irradiance decreases exponentially with increasing depth • Light becomes limiting factor to photosynthesis Compensation Depth • Compensation depth – the depth at which the amount of oxygen produced in photosynthesis equals the oxygen consumed in respiration Net increase of oxygen over time COMPENSATION DEPTH Net decrease of oxygen over time D E P T H Compensation Depth • Is controlled by season, latitude, and transparency of water column – Longer photoperiod in temperate-boreal waters – Arctic winter has a zero photoperiod – Suspended matter in coastal waters intercepts light Photosynthesis and Light Intensity Before the Spring Phytoplankton Increase • In winter: – Water density is similar at all depths – Wind mixing homogenizes water column – No bloom because any potential profit in photosynthesis would be lost to mixing Seasonal Changes in Mixing and Light Water column stability is essential to the development of the spring bloom Key Processes Leading to Spring Phytoplankton Increase Key processes: • Development of thermocline • Trapping of nutrients • Retaining of phytoplankton Spring Bloom in the Gulf of Maine Decline of the Spring Phytoplankton Bloom • Nutrients are being removed from stable water column • No replenishment of nutrients from deeper water • Zooplankton grazing has some effect but is often secondary to sinking Rejuvenation of Conditions for the Spring Phytoplankton Increase • In fall and winter: water cools, water column becomes isothermal with depth, wind mixing restores nutrients to surface waters until conditions are right next spring Water Column Exchange in Shallow Waters and Estuaries • Importance of water column stability varies with basin depth and season • Benthic-pelagic coupling – nutrient exchange between the bottom and the water column – Fuels more phytoplankton growth Water Column Exchange in Shallow Waters and Estuaries Benthic-Pelagic Coupling and a Beach Bloom Water Column Exchange in Shallow Waters and Estuaries • High primary production in estuaries • Nutrient regime is determined by the combination of the spring freshet with mixing and net water flow to the sea Important Factors in Water Column Exchange in Shallow Waters and Estuaries • Residence time - time water remains in estuary before entering ocean • Rate of nutrient input from watershed • Nutrients may be released to coastal zone Nutrients • Nutrients are dissolved or particulate substances required by plants and photosynthetic protists; • Can be limiting resources Nutrients in Marine vs. Terrestrial Environments Terrestrial • Agricultural soil = 0.5%N • Allows for greater primary production per m3 • Long-lived plants Marine • Ocean waters = 0.00005%N • Allows for much less primary production per m3 • Short-lived plants • Nutrients are often limiting Nitrogen – New vs. Regenerated Production • New production: – Nutrients for primary production may derive from input of nutrients from outside the photic zone • Regenerated production: – Nutrients derive from recycling in surface waters from excretion Phosphorous (P) • P is rapidly recycled between water and phytoplankton • Sediments accumulate P from phytoplankton detritus • Diffusion of P from bottom due to benthic decomposition • Winter mixing returns P to surface waters N and P as Limiting Nutrients • N and P are depleted by phytoplankton growth • Phytoplankton more enriched in N than P, suggesting that N is limiting to primary production on the scale of the entire ocean • P ultimately comes from weathering of minerals Silicon • Important limiting element for diatoms • Sinking of diatoms from surface waters removes silicon • Silica (Silicon dioxide) delivered to ocean by wind and river transport Fe, Si often enter the ocean by windborne particles Iron as a Limiting Nutrient and in Climate Change • Is commonly in short supply and is thus limiting to phytoplankton • May be crucial in parts of the ocean where nitrogen appears not to be limiting factor (HNLP zones) • Phytoplankton sequester large amounts of CO2 during photosynthesis • Dr. John Martin – Idea was that if you increase phytoplankton production, you could slow global warming • Evidence – Eruption of Mount Pinatubo in 1991 IronEx Studies • IronEx I (1993) – First open ocean iron fertilization experiment – Single iron addition to a 100 km2 patch of water near Galapagos Islands – Results not very dramatic – Proved that iron can limit primary production in some of the world’s oceans • IronEx II (1995) - Sequential additions of solubilized iron to water patch in Equatorial Pacific – Produced enormous phytoplankton bloom • Have been 13 iron fertilization experiments since 1993 Intense and Harmful Algal Blooms • Conditions: 1. A stable water column 2. Input of nutrients 3. Sometimes an initial input of resting stages • Principally some dinoflagellates and cyanobacteria • Population crashes may reduce oxygen in water Red Tide off Florida Coast Phytoplankton Succession • Seasonal change in dominance by different phytoplankton species • General properties correspond to the seasonal trend in nutrient availability Phytoplankton Succession Mechanisms poorly understood: • Shift in advantage of nutrient uptake, later species in season may depend upon substances that are not in the water column in early spring • Stratification • Chromatic adaptation • Allelopathy Paradox of the Plankton - Hutchinison • Coexistence of many photosynthetic and heterotrophic groups under nutrient limitation • Would expect an equilibrium would be reached and one species would dominate • Remains to be solved, but could be due to: – Spatial patchiness of nutrients – Reproductive capacity of phytoplankton 3 Major Pathways for Flux of Organic Matter • Grazing food chain • Microbial loop • Sinking flux The Microbial Loop 1. Bacteria take up large amounts of nutrients and organic matter from the water column 2. Bacteria are consumed by ciliates and other heterotrophs 3. These heterotrophs are consumed by other smaller zooplankton The Microbial Loop DOC=dissolved organic carbon POC=particulate organic carbon DIOC=dissolved inorganic carbon The Microbial Loop and Deepwater Horizon • Bacteria in microbial loop feed on oil droplets (is a source of C) and associated contaminants • May alter microbial loop and its functioning • Allows contaminants to enter planktonic food web and reach higher order consumers Marine Snow • Fragile organic aggregate made up of dissolved organic molecules or degraded gelatinous substances • Usually enriched with microorganisms • Found in relatively quiet water Zooplankton Grazing • Zooplankton growth depends on phytoplankton growth • Zooplankton abundance usually increases after the peak of phytoplankton abundance • Grazing effect: Difference between grazing rate and phytoplankton growth rate • Grazing quite variable North Sea: grazing results in alternating patches of phytoplankton and zooplankton cycles of abundance Zooplankton Grazing Copepod feeding response to diatom density Zooplankton Feeding/Grazing • Zooplankton can select phytoplankton particles by size • Could influence species composition of phytoplankton Diurnal Vertical Migration of Zooplankton • Rise to shallow water at night, sink to deeper water during the day Planktonic shrimp, Sergia lucens Causes of Diurnal Vertical Migration of Zooplankton • Strong light hypothesis – plankton are adversely affected by UV radiation and strong light, so they migrate away from surface waters during the day • Problem? Causes of Diurnal Vertical Migration of Zooplankton • Phytoplankton recovery hypothesis - zooplankton migrate downward to allow phytoplankton to photosynthesize and recover during the day • Problem? Causes of Diurnal Vertical Migration of Zooplankton • Predation hypothesis - zooplankton migrate downward to avoid visual predation during day • Problems? Causes of Diurnal Vertical Migration of Zooplankton • Energy conservation hypothesis - zooplankton migrate downward to avoid higher surface temperatures during the day, which saves energy (metabolic rate and energy needs are lower in cooler waters) • Problem? Defenses Against Predation • Body spines • Being nearly transparent • Bioluminescence – Counterillumination – Deceptive signals – Camouflage – Lures • Toxic substances