Epiphyte Growth, Water
Quality, and Submersed Aquatic Vegetation in the Tidal Potomac River and
Estuary
The relationships among epiphyte colonization, water quality, and submersed aquatic vegetation were examined using artificial substrates in the tidal Potomac River and Estuary. Clear mylar strips suspended in the water column were used to simulate plants at two sites in the freshwater tidal river and two sites in the mesohaline portion of the Potomac Estuary. Each month throughout the growing season strips were placed at each site and left from 11 to 29 days. Epiphytic dry weight, biomass, chlorophyll-a, and light transmission reduction were determined for the strips. In addition, water-quality measurements were taken every two weeks for total suspended solids, total suspended volatile solids, chlorophyll-a, light attenuation, and dissolved inorganic nitrogen and phosphorus at each site throughout the experiment. While positive correlations among epiphytic light transmission reduction, dry weight, and chlorophyll-a were found, there were no significant correlations between nutrients and epiphytic light transmission reduction, possibly because nutrients were not limiting. Mean light transmission reduction due to the presence of epiphytes on the strip was more limiting at the mesohaline sites, at 77%, than at the freshwater sites at 46%. A coefficient of light transmission reduction was calculated based on the mesohaline data. These data are being used to calibrate models involving the effect of epiphytic light transmission reduction on submersed aquatic vegetation growth.
As part of a recent study, U. S. Geological Survey (USGS) researchers measured epiphyte colonization, and water quality throughout the growing season at sites in the fresh and mesohaline tidal Potomac River and Estuary to examine their significance to the propagation of submersed aquatic vegetation (SAV). Epiphytes, which coat the leaves of submersed aquatic plants, can be made up of algae, sediment from the water column, bryophytes, barnacles, insect larvae, and other micro and macro organisms. The amount of light a plant leaf has available for photosynthesis is affected not only by water column light attenuation but also by the amount of epiphytes present on the leaves. Light transmission reduction due to epiphytes, epiphytic dry weight, biomass, and chlorophyll-a (chl-a) content were measured on artificial strips which simulated SAV. Water-quality measurements were made at the sites to examine the relationship of water quality to epiphytic colonization and transplant survival.
Clear mylar strips were used to simulate SAV in order to measure light transmission reduction due to epiphytes as well as epiphytic dry weight, biomass, and chl-a. The strips were placed on a hanger with weights to maintain a general vertical orientation. On each hanger there were twelve strips. Two hangers were placed at each of four sites on the Potomac, two on the freshwater tidal river, and two in the mesohaline estuary.
Figure 1. Epiphyte Artificial Substrates (designed by University of Maryland Center for Environmental Science, Chesapeake Bay Lab)
Figure 2. Below is a map of the Chesapeake Bay Program salinity zones for the tidal Potomac River and Potomac Estuary. Study site names in black correspond to epiphyte strip hanger locations. The mesohaline sites are also USGS water-quality collection, transplant, and propagule flux study sites. Station names in red correspond to relevant Maryland-Department of Natural Resources (MD-DNR) mainstem water-quality monitoring stations.
The strips were analyzed
for light transmission reduction, chl-a, epiphytic dry weight, and epiphytic
biomass. For analysis the top and bottom 2.5 inches were not considered
as they are handled during processing. The light, or Photosyntheticly
Active Radiation (PAR), transmission reduction was measured underwater
using a Li-cor sensor and data logger. To simulate the relative amount
of light available to a submersed aquatic plant leaf, one side of the strip
was scraped clean using a razor blade. The amount of light passing
through the epiphytes and the strip was then measured and corrected for
the reduction due to the strip itself. Three strips were analyzed
for each site and on each strip three Li-cor readings were taken at the
top, in the middle, and at the bottom for a total of nine readings per
strip and 27 per site. Chl-a was measured by cutting the sample volume
of the strip up into two inch pieces and doing an acid extraction to measure
chl-a, active chl-a, and phaeophyton. Epiphytic dry weight was measured
by scraping a subsampling of the strips with a razor blade at the top,
middle, and bottom. Biomass was measured by the total volatile solids
content of the epiphytic total dry weight.
Total dry weights were
correlated to PAR transmission reduction to give a coefficient, which
can predict reduction based on a given dry weight. The formula ln(Io/Ie)
= KeBde (Kemp & Bartleson, written communication July, 1998) where
Io is the PAR transmission without epiphytes, Ie is the PAR transmission
with epiphytes present, Ke is the coefficient of PAR transmission reduction,
and Bde is the total epiphytic dry weight, was used to estimate the PAR
transmission reduction based on epiphytic dry weight.
Figure 3. PAR transmission reduction over time is much higher at the mesohaline sites and peaks in late summer.
Figure 4. Epiphytic total dry weight over time shows similar trends to PAR transmission reduction at the mesohaline sites but show weak PAR transmission reduction trends at the freshwater sites.
Figure 5. The mesohaline sites show a strong correlation between PAR transmission reduction and epiphytic dry weight and the freshwater sites have insignificant correlations, possibly in part due to their low overall epiphyte weights. The lines are forced through zero.
Figure 6. The epiphytic total dry weight is almost entirely organic which indicates that inorganic material from suspended sediment does not make up a major portion of the dry weight. While there was a positive correlation between biomass and chl-a in the mesohaline; the correlation between the two at the freshwater sites was insignificant.
Figure 7. This figure also illustrates that epiphyitc organic dry weight is not well correlated with chl-a.
The effect of epiphytes on PAR transmission reduction has been previously studied by other researchers with various methods and conditions using both dry weight, biomass, and chl-a as predictors. Our Ke value (Ke= 0.18) using epiphytic dry weight data was low compared to literature values. Literature coefficients range from 0.20 to 0.50 depending on the investigation. The low chl-a component of these epiphytes as well as other sites specific conditions could explain the low rate of PAR transmission reduction. Using chlorophyll-a as an indicator of PAR transmission gave coefficients much higher than cited values. The correlations were weak and nonlinear suggesting that algae were not a major component of the epiphytic dry weight or biomass.
Water-quality measurements were taken in the tidal freshwater Potomac river and mesohaline Potomac Estuary at and near the study sites (DM, DD, LM, YO) every two weeks throughout the growing season to test for correlations among artificial substrate epiphyte data and nutrients, water column light, chl-a, and suspended solids. The measurements include nitrite, nitrate, dissolved inorganic nitrogen, dissolved inorganic phosphorus, total suspended solids (TSS), total volatile solids, chl-a, active chl-a, phaeophyton, light extinction, pH, dissolved oxygen, salinity, conductivity, and temperature.
There were no meaningful correlations between any water-quality parameters and epiphyte data that could be applied to both the fresh and mesohaline sites. Several mesocosm experiments have addressed the effects of nutrients on epiphytes of several different SAV species. They show that increased N and P in the correct ratio can increase epiphyte growth but few field exist. This may be because in this and other situations no one water-quality parameter was significantly limiting. Water column light attenuation can have a varied effect on the amount and composition of epiphytes. In relatively clear water epiphytic algae can flourish, while in more turbid water other non-photosynthetic epiphytes can grow on plants, and suspended solids may be deposited.
Mean PAR (or light)
transmission reduction due to the presence of epiphytes was 46% at the
freshwater and 77% at the mesohaline sites. A PAR transmission reduction
coefficient was created for the mesohaline using the epiphyte dry weight.
At 0.18 the coefficient for Ke using epiphytic dry weight as the input
is low when compared to values based on other data sets (0.50-0.20).
Although some significant
correlations between epiphytic light attenuation, total dry weight, organic
dry weight, and chlorophyll were found, there were no significant correlations
between nutrients and epiphytic light attenuation, possibly because nutrients
were not limiting.