Midsummer emergence of chironomid taxa in streams spanning a gradient of watershed urbanization

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Mid-summer emergence of chironomid taxa in
streams spanning a gradient of watershed
urbanization
Susan E. Gresens
Alyson Paul
Towson University Department of Biological
Sciences 8000 York Road Towson, MD 21252-0001
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How does species diversity change along a rural
to urban gradient?
or ?
(McKinney 2002)
rates of population growth
Species diversity
disturbance
Dynamic equilibrium hypothesis (Huston 1979)
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Model system Chironomid (Diptera) assemblages in
small streams
egg
larva
pupa
adult
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Method analysis of surface-floating pupal
exuviae -- most efficient for species
identification -- spatially integrated sample
from a stream
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Taxonomy application in bioassessment
1300 species in Europe!
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Survey of 25 stream sites during 2002
Watershed landuse range from forest to urban
Sampling at 3 week intervals Feb. to Aug. Nov.,
Dec.
ORBR
BAIS
GDWN
TIMB
DEEP
TOWS
CHIM
HRRG
WHTM
STON
PDRM
MOOR
Environmental variables substrate texture, water
chemistry, nutrients, canopy cover, algal biomass
DEAD
ARMI
MDCH
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Large numbers of exuviae collected from forested
streams
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Initial analysis -- focus on 11 easily
recognized species/genera -- mid-June samples
from15 stream sites analyzed to date
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How is the distribution of chironomid taxa
related to environmental variables?
Canonical correspondence analysis (CCA)
-- Ordering of taxa / sites is constrained by
their relation to measured environmental variables
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What environmental gradients are represented by
this set of stream sites?
16 inertia
Site scores
38 inertia
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Rural ? ? ? ? ? urban
Coarser substrates Higher phosphorus Greater
peak algal biomass Higher Zn, Pb conc.
More fine sand, silt substrates Lower
phosphorus Lower conductivity
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Response of taxa to environmental gradients
16 interia
Taxa scores
38 interia
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Responses of chironomid taxa
-- Optimal habitats for Tanytarsini species
streams with fine sediments, watersheds with
greater forest cover
Chironomids suggest not all urban streams are
equal -- Cricotopus, Chironomus tolerate
higher Zn, Pb conc., associated with high algal
biomass (Cladophora) -- Cryptochironomus,
Conchapelopia, Paratanytarsus nr. inopertus more
tolerant of eutrophication, (perhaps greater
substrate disturbance?)
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Conclusions
-- even this small number of taxa show a wide
variety of responses to the effects of
urbanization -- chironomid exuviae may provide
ability to discriminate stressors in
bioassessment of urban streams -- algal biomass
and substrate data suggest that, across these
streams, gradients of algal productivity and
physical disturbance are probably not independent
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Values of environmental variables at study streams
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Distribution of Cricotopus and Tanytarsus species
across the gradient of urbanization