Biofilm biomass and nutrient uptake at Smith Creek

1
Biofilm biomass and nutrient uptake at Smith
Creek Kevin Simon1 and Beth Cheever2 1School of
Integrative Biology and Ecology, University of
Maine, ksimon_at_maine.edu 2Department of Biology,
Virginia Tech
1. Background
3. Biofilm Biomass
One of the most common results of agricultural
landuse is the increased input of nutrients to
streams . Increased nutrient delivery to streams
can stimulate productivity, but excessive
nutrient enrichment degrades water quality and
habitat, both locally and in downstream systems,
especially lakes and estuaries. We measured the
biomass of biofilms on rocks in multiple reaches
of Smith Creek as a rough indicator of
productivity. We also measured nutrient uptake
at the reach scale in 3 reaches of the Smith
Creek drainage. In general, agricultural landuse
is expected to increase biofilm biomass,
particularly the non-algal component of it.
Agricultural landuse generally leads to increased
rate of nutrient uptake as nutrient availability
increases, but reduced nutrient retention and
nutrient uptake efficiency.
Algal biomass was lowest in the shaded reach of
Mountain Run and in the Spring Run feeding Smith
Creek. Algal biomass was about twice as high in
the mixed use zone than in the shaded reach.
Algal biomass was highest in Bruce Farm reaches.
mg cm-2
Total biofilm biomass was similar in the shaded
reach, mixed use zone and spring run. In
Mountain Run on the Bruce Farm total biomass was
generally higher than at the other sites.
mg cm-2

• 2. Methods
• Biofilm Biomass
• Rocks were collected from each site during
  invertebrate sampling. For each sample, a single
  rock (n6 per site) was collected, placed into a
  plastic bag, and frozen until analysis.
• Biofilm biomass on rocks was quantified as
  chlorophyll a (an indicator of algal biomass) and
  Ash Free Dry Mass (AFDM, the total amount of
  organic matter on the rock, algal and otherwise).
  For analysis, biofilm on the rocks was scrubbed
  and filtered onto glass fiber filters.
  Chlorophyll a on the filters was extracted using
  hot ethanol and measured spectrophotometrically.
  AFDM was calculated by drying the filters,
  weighing them, combusting the organic matter on
  the filters at 500C and then reweighing the
  filters. The relative contribution of algae to
  biofilm biomass was determined as an autotrophic
  index (AI, the ratio of AFDM to chlorophyll a).
• Nutrient Uptake
• Uptake of 3 nutrients (NH4, NO3- and PO43-)
  in reaches of the stream in the mixed use zone
  and the upper and lower segments of Smith Creek
  on the Bruce Farm were measured by adding the
  nutrients and a conservative tracer (Cl-, used to
  account for groundwater influx to the reaches) to
  250 m reaches of the streams. The longitudinal
  decline in nutrients along the reaches was used
  to calculate three metrics of nutrient uptake
• Uptake Length the average distance a nutrient
  travels downstream before it is taken up on the
  streambed. Long uptake lengths low nutrient
  retention. Note that uptake length is strongly
  influenced by stream size (larger streams
  generally have longer uptake lengths because they
  are deeper and faster).
• Uptake Rate the rate of removal of nutrients
  from the water column per unit are of streambed.
• Uptake Velocity the ratio of uptake rate to
  nutrient concentration in the stream. This is an
  indicator of uptake efficiency (i.e. how large is
  uptake rate relative to nutrient availability?).

There were no strong patterns in the relative
contribution of algal to biofilm biomass among
sites.
4. Nutrient Uptake
NO3 concentration was an order of magnitude
higher on the Bruce Farm than in the mixed use
zone. NO3 retention was low in all 3 sites (NO3
traveled 5-10km on average before uptake). NO3
uptake was more efficient in the mixed use and
Bruce Upper reaches.
NH4 concentration was reasonably low and similar
among reaches. NH4 uptake was generally similar
among reaches.
PO4 concentration was elevated in all reaches
compared to that in the forested reaches of
Mountain Run. Uptake efficiency was highest in
the Bruce Lower reach.
5. Conclusions
Higher nutrient availability and light have
likely led to higher biofilm biomass in the lower
reaches of the Smith Creek drainage. Most of the
biofilms are composed of dead organic matter
rather than algal films. Nitrate uptake
parameters are typical of a stream system that is
being saturated with nitrogen, with low retention
and uptake efficiency of nutrients. Despite
having some riparian vegetation, the mixed use
zone is not extremely different from the lower
reaches of Smith Creek in terms of nutrient
uptake, although biofilm biomass is more similar
to that expected for a forested system.