OCEAN 320 Lecture 3: Unit 3C
Unit 3C: Functioning of Marine Ecosystems
• Ecosystem: “a spatially explicit unit of earth that includes all of the organisms, along
with all the components of the abiotic environment within its boundaries
o Vague; ecosystems have no apparent boundaries and lack clear objective/purpose
o Components of ecosystems are connected in complex food web by evolving
interactions
• Fisheries management largely based on single species approached; ecosystem
management represents paradigm shift, new attitude towards exploitation of renewable
marine resources
o Viewed as an integrative level for ecological studies, complexity is critical to
sustainability
o Important to understand major structural changes and how reversible those
changes are
• Marine ecosystems function on structure, diversity, and integrity; alteration/disturbance
have strong effects on high/low trophic levels, depending on how food webs are
controlled (by predator or prey/resources?)
• Victor Hensen (1887): thought of planktonic populations as rapidly revolving links in
food chain from very small to very large
o Food supply regulates adult fish sticks & more phytoplankton/zooplankton=
higher fish yields
o Ecosystems are “bottom-up” controlled; food web components derive from
primary producers/limited nutrients
o Nutrients enhance phytoplankton biomass; ocean’s animals are fed by thin soup
of minute algae, which controls global productivity
o Marine environment is a dispersive and heterogenous on; species are not evely
distributed spatially; also fluctuate widely from year to year
o Renewable processes in fish population dynamics are highly irregular, depend on
recruitment strength, and marine fish species comprise many self-sustaining
populations
o Considerable evidence than natural variability in ocean circulation and mixing
plays major role in generating fluctuations in marine productivity & distribution
of populations
o Food availability and physical constraints (retention/concentration/enrichment
caused by currents/turbulence) are as important factors as larval survival,
recruitment, stock abundance
• Structure & function respond drastically to inter-annual changes and inter-decadal
climatic variations
o Ex: California Current, Gulf of Alaska, North Atlantic, off Chile; parallel long
term trends across 4 marine trophic levels, related to environmental changes in the
North Sea
o Using trophic mass-balance models: multiple & complex changes of th Bering
Sea (1950’s-80’s) also largely due to environmental changes
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▪ Inter-annual environmental fluctuations (El Nino) affected structure of
plankton community, spatial distribution of fish and invertebrates,
recruitment success of pelagic fish and mortality of birds and mammals in
northern Pacific
o Inter-decadal regime shifts, ie: north Pacific Basin and California Current, appear
to have altered productivity of marine ecosystems at various trophic levels
▪ Increase in southern species moving north, substantial lowering of
secondary productivity and fish landings, major decline in seabirds,
changes in species composition in most sectors of ecosystems
▪ Biological response to inter-decadal regime shift in Gulf of Alaska thought
to have been in opposite direction of California Current
▪ Large scale biological response to low-frequency climatic variation
▪ Mechanisms by which climate exerts influence vary as components of
ecosystem are constrained by different limiting environmental factors
▪ Similar species at same trophic level may respond differently to climate
change; one system will not react exactly like others, hard to predict
effects of global environmental change
• Large fluctuations for pelagic fish occur even in absence of fisheries; roves that
overfishing is not only cause of fluctuations
o Sediment records used to determine salmon abundance over past 300 years; some
pronounced changes due to climatic change
o Regime shifts alter nutrient cycles and have significant impact on productivity of
ecosystems
o Fish populations also show global synchrony; most likely driven by global
climatic teleconnections; changes in abundance of prey has major consequences
o Small, pelagic fish are forage fish; they are food sources for top predators;
collapse of prey species (induced by climate or fisheries) causes huge blows for
all their predators and species at their same trophic levels
• Alternating steady states also observed on decadal scales
o Ex: upwelling systems usually dominated by one species of sardines and one
species of anchovy, but only one of the two is dominant at any time; alternating
patterns observed in most upwelling ecosystems
o Regime shift between two species occurs when, after removal of species, biomass
is restored by density compensation of other species
▪ May occur between two redundant species (from same guild or functional
group)
o Dominant species responds to environmental factors, while subordinate species
responds to abundance of dominant species
o Climatic factors thought to affect fluctuations in abundance, whereas absolute
density is controlled by intraspecific competition
o Under bottom up control: physical environment drastically affects overall
productivity and dynamics of fish assemblages in more or less predictable way
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