Lakes as records of human impacts

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Lakes as records of human impacts

• Michael de Sousa
• Bio. 402

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Paleolimnology

• Paleolimnology is the study of past water
  environments.
• A core is taken from the sediments of a system,
  analyzed for aspects such as
• Ecological
• Geographical
• Hydrological
• Zoological
• biological

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Paleolimnology

• Signals preserved in the core are the clues
  revealing the ecological history of the lake and
  surrounding landscape
• These clues can result in quantitative and
  qualitative reconstructions of lake chemistry and
  biology or the past.

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Paleolimnology

• Such a stratiagraphy is composed of a
  chronological succession in various climatic,
  environmental and anthropogenic impacts
• Key is to recognize the various events along the
  geologic timeline and to date these with best
  possible methods

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Paleolimnology

• Proxies include
• Pollen
• sediment grain size
• Geochemistry
• magnetic susceptibility
• To infer
• Precipitation
• Temperature
• nutrient availability
• storminess

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Prehistoric Inuit whalers affected Arctic
freshwater ecosystems

• Douglas et al. 2004

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Thule Inuit

• Thule Inuit first whalers in High Arctic
• Migrated to Canadian Arctic and Greenland from
  Alaska 1000 y.a
• Brought with them well developed whaling
  technology.

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Somerset Island

• Greatest concentration of whale bones in Canadian
  Arctic occur on Somerset Island.
• Aerial surveys bleached bones and lush
  vegetation as it continues to be fertilized by
  decomposing bones and other refuse

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Study Site

• On the coast 5km North of Hazard Inlet entrance
• 11 whale bone houses, smaller dwellings, and ring
  of 10 bowhead crania
• Occupation from 13th to the end of the16th
  century.

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Study Site

• Differed from hundreds of other ponds surveyed
  without remains
• uncharacteristically high P, organic C and Ca
• Sampled ponds with whale bones displayed high
  concentrations of similar nutrients.

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Field Methods

• Sediment cores from central region of pond all 23
  cm in length
• pond contained 23 cm of sediment confident cores
  represent the entire sedimentary history

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Geochronology

• Pb dating
• Archeological materials (antlers) dated by
  accelerator MS radiocarbon dating

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Analysis

• Diatom taxa were identified and enumerated
• Sediment samples were analyzed for their nitrogen
  stable isotope ratio

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Results and DiscussionGeochronology

• Extrapolation of slower sedimentation rates
  indicated anno Domini (A.D. 1600) at 12cm
• Extrapolation of the Pb derived rates places
  circa (ca) A.D. 1200 at 19cm, coinciding with
  diatom and N isotope changes
• Dating of antler debitage and artifacts all
  indicate ca. A.D. 1200-1600
• All dating approaches are consistent with the
  chronology and interpretations suggested for this
  sediment core.

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Paleolimnological indicators

• Earliest assemblages dominated by F. pinnata
• Unlike other Arctic ponds, Pinnularia
  balfouriana, characterized the flora at 19cm
  level (ca. A.D. 1200)
• Peaked at 11-12cm (ca. A.D. 1600)

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Paleolimnological indicators

• Timing of changes coincides with Thule
  settlement ecology was dramatically influenced
• Decaying bones fertilized the catchment,
  promoting moss growth
• Increased nutrients would affect diatoms, and
  moss proliferation would provide a substrate for
  diatom growth

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Marine Biomass

• N average 13 for bowhead whales greatly exceed
  those normally found in most freshwater systems
• N values in other Artic sediments typically fall
  within 0-3
• Rise in N from 2.5 to 5.4 at 20cm is not
  typical of sediment cores from Arctic ponds
• Confirms a marine input signal from decomposition
  of bowhead remains

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High Nutrients

• Site records recent assemblage shifts and higher
  nutrient conditions than those in other Arctic
  ponds not influenced by Thule.
• Warming after the mid-19th century transport of
  nutrients from remaining whale bones may
  accelerate, increasing the input of nutrients.

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Conclusions

• Humans affected aquatic ecology much earlier than
  previously thought
• Ironic that the High Arctic, considered to be the
  last refuge from human disturbances
• Contains oldest record thus far in the U.S. or
  Canada of a human population affecting the
  freshwater ecology

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History and timing of human impact on Lake
Victoria, East Africa

• Verschuren et al. 2002

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Lake Victoria

• Lake Victoria has undergone
• Increase in phytoplankton PP
• Replacement of diatoms by cyanobacteria
• Eradication of several hundred species of endemic
  cichlid fish and explosion of Nile perch

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Correlation

• Coincidence in timing of cyanobacteria blooms
  with collapse of indigenous fish has tempted the
  question
• To what extent does the decimation of
  phytoplanktivorous cichlids by Nile perch
  contribute to increased algal production?

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Methods

• In 1995 and 1996 they recovered surface-sediment
  cores from six offshore stations between 48 and
  68 m water depth
• Environmental history over the past 180 years as
  recorded in core V96-5MC from the deepest part of
  the lake

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Methods

• Water content and dry mass determined by drying
  for 20 h at 105 C
• Sediment age at depth and accumulation rates
  determined by Pb dating
• Silica in the sediments were analyzed by wet
  alkaline extraction
• Dissolved Si in pore water was analyzed by
  spectrophotometry
• Fossil diatom assemblages were analyzed
  quantitatively

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ResultsOffshore sedimentation dynamics

• Unsupported Pb 210 activity decreases
  exponentially with cumulative dry mass downcore
  indicates sediments have accumulated at a
  relatively constant rate (2.3mm/yr)
• Shift to lower Pb activity at depth of 25-26 cm
  reflects a disconformity interpreted as sediment
  erosion during an exceptionally violent storm
• Only deepest part of the lake is wave-induced
  erosion rare sediment record preserved
  sufficiently intact to reconstruct history

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Historical diatom production

• High Si concentrations deposited from the 1930s
  to 1980s reflect increased nutrient supply
• Loss of excess Si to burial in deep-water
  sediments gradually depleted Lake reservoir of
  dissolved Si.

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Historical diatom production

• Concentrations have declined from 70-80
  micro-molar in 1960 to 8 in 1990 frequently
  below 1
• Lower diatom-Si concentrations deposited since
  late 1980s reflect situation in which diatom
  production is limited by the dissolved-Si
  concentration realized through recycling of Si
  from shallow-water sediments and new inputs from
  the watershed through soil runoff

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Historical diatom production

• Between 1820 and 1940, Cyclostephanos and
  Aulacoseira co-dominant and Nitzschia acicularis
  absent
• All three diatom taxa increased from 1940 to
  1960s Nitzschia acicularis 50 of fossil
  assemblages dominance not observed at any
  previous time during the past 12 400 years.
• Fossil-diatom record then indicates reduction of
  diatom production in late 1980s
• Matches occurrence of massive cyanobacteria
  blooms after 1987
• Indicates restructuring of the algal community to
  cyanobacteria dominance in mid-1980s was
  influenced by Si limitation of diatom growth, due
  to seasonal depletion of a diminished
  dissolved-Si reservoir

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Development of deep-water anoxia

• Changes in abundance ratios indicate after 140 yr
  of year-round bottom oxygenation, oxygen regime
  of started to deteriorate in 1960s
• Persistent deep-water anoxia appears to have
  reached its current spatial extent by the late
  1970s
• Fossil data agree with fragmentary historical
  dissolved-oxygen data from the 1920s, 1960s and
  1990s implying intermittent deep-water anoxia
  first observed in 1960 represented the earliest
  stage of eutrophication-induced deep-water oxygen
  loss in Lake Victoria

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Discussion

• The combined fossil evidence indicates that
  historical changes in phytoplankton productivity
  and composition of Lake Victoria have been caused
  mainly by bottom-up effects of excess nutrient
  loading, and less so by food-web alterations
  after the 1980s upsurge of Nile perch

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Discussion

• Timing and progress of inferred productivity
  increases match human-population growth and
  agricultural activity
• Used population size with agricultural production
  over past 40 years as an indicator for soil
  disturbance and its effect on nutrient fluxes
• Population went from 4.6 million in 1932 to 27.7
  million in 1995

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Discussion

• Experimental trawls since early 1970s does not
  show evidence that expansion of deep-water anoxia
  caused cichlids to migrate to shallower depths
• Anoxia must have contributed to demise of certain
  haplochromine taxa dependent on the deep-water
  mud habitat, but it probably did not trigger the
  population explosion of Nile perch

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The Future

• Projecting doubling of regional human population
  to 53 million by the 2020, further degradation
  can be countered only if land-management
  strategies restricting nutrient inputs are
  implemented