s, tiff ~ ~~iil r Circular 215 June 1975 for (Iompitii . $ lilit Piii 4, inl Iis11 1onlds AGRICULTURAL EXPERIMENT STATION AUBURN UNIVERSITY R. Dennis Rouse, Director, Auburn, Alabama CONTENTS page INTRODUCTION-------- ----- -- -- ---- -- - -- 3 MATERIALS AND METHODS----------------- 4 Effect of Light Intensity on Photosynthesis by A quatic W eeds ------- ----- ----- ------ ------ 4 Competition Between Phytoplankton and Underwater W eeds--------------------- --- -5 Ecological Effects of Duckweeds- -- 6 RESULTS AND DISCUSSION-------------- ---- 6 Effects of Light Intensity on Photosynthesis by A quatic W eeds------------------------------------- 6 Competition Between Phytoplankton and U nderw ater W eeds---------------------------------- 9 Standing Crops of Aquatic W eeds----------------------12 Ecological Effects of Duckweeds-----------------------13 S U M M A R Y -- - - - - - - - - - - - - - --- - -- - -- - - - - - - - - - - - - - - - - - - - - - 18 LITERATURE CITED----------------------------------- 18 FIRST PRINTING 3M, JUNE 1975 Auburn University Is An Equal Opportunity Employer [2] COMPETITION FOR LIGHT BY AQUATIC PLANTS Claude E. Boyd* INTRODUCTION U NMANAGED PONDS in most areas of Alabama often have rela- tively clear water and are therefore excellent habitats for growth of underwater and emergent plants. These plants cause unbal- anced fish populations, interfere with fishing and fish harvest, favor mosquito production, and compete with phytoplankton for light and nutrients (5). Early studies on fertilization of fish ponds revealed that additions of inorganic nutrients increased phyto- plankton production and fish yield. The resulting phytoplankton turbidity also shaded and eliminated underwater weeds from ponds (15, 16). Small, sheltered fish ponds with waters rich in organic matter are often infested with duckweeds (6,8,13). These plants grow on pond surfaces and interfere with gas exchange between water and atmosphere, inhibit photosynthesis by underwater plants, and cause low oxygen tensions in waters beneath (7). Fertiliza- tion is not effective in controlling duckweeds. The early studies of pond fertilization were primarily concerned with immediate means of increasing fish production, but through necessity, a number of practical generalizations were made on the ecology of aquatic weeds in ponds (14,15,16,18,19). Unfor- tunately, many of the observations on the ecology of aquatic weeds have never been substantiated with data and are open to criticism. The present report contains data on competition be- tween aquatic plants for light and the influence of this competi- tion on the pond environment. a Associate Professor, Department of Fisheries and Allied Aquacultures. MATERIALS AND METHODS Effect of Light Intensity on Photosynthesis by Aquatic Weeds Nutrient solutions were prepared from well water which had been heated with stirring to reduce its dissolved oxygen content. Lowering of the dissolved oxygen content was necessary to pre- vent supersaturation of water with dissolved oxygen during photo- synthesis experiments (10). After cooling, ammonium nitrate and potassium dihydrogen phosphate were added to raise concen- trations of nitrogen and phosphorus to 1.0 and 0.25 parts per million, respectively. Available carbon in different batches was similar; pH (6.95 to 7.25), total alkalinity (41.7 to 43.8 parts per million), and carbon dioxide (5.3 to 8.4 parts per million). Dis- solved oxygen concentrations in different batches ranged from 4.0 to 5.5 parts per million. An analysis of well water by atomic absorption spectrophotometry gave the following results in parts per million; calcium 4.0, magnesium 2.8, sodium 6.5, potassium 1.1, iron 3.4, manganese 0.05, zinc 0.29, and copper 0.05. Aquatic weeds were collected from depths of 1 to 3 feet in ponds on the Fisheries Research Unit of the Auburn University Agricul- tural Experiment Station, Auburn, Alabama. In each series of photosynthesis measurements, fresh 1.0-gram samples of plants were placed in each of 15 standard BOD bottles. The bottles were put in a dark box and transported to a nearby pond. Nutri- ent solution was introduced through a siphon tube which dis- charged at the bottom of bottles. Bottles were allowed to over- flow for one complete exchange of water, stoppered, and returned to the dark box. Eleven BOD bottles were attached horizontally by universal clamps at 10-inch intervals along a metal rod. The rod was then suspended vertically so that bottles were positioned at 10-inch intervals from the surface to 8.3 feet. Two bottles (dark bottles) were wrapped in aluminum foil and suspended at depths of 1 and 4 feet. As soon as light and dark bottles were in place, nutrient solution from the two remaining bottles (initial bottles) was siphoned into 60-milliliter bottles for dissolved oxygen determinations by the Winkler method (1). Transfer of plant material was prevented by fitting a piece of bolting silk over the siphon intake. After 1-hour incubation, light and dark bottles were quickly placed in the dark. Bottles were then re- moved one at a time and nutrient solution siphoned to 60-milli- liter bottles for dissolved oxygen analysis. Dark bottles were always siphoned last and the time of dark storage (5 to 15 [4] minutes) between the end of incubation and siphoning recorded. Rates of respiration and net photosynthesis per gram fresh plant material were calculated from the following equations. (1) Respiration (iuMO 2/hr) = MO 2 I.B. - /MO 2 D.B. 1hr. + t (2) Net photosynthesis (ILMO 2/hr) = , MO2 L.B. - MO 2 I.B. + (t) (Respiration) Where: ~MO 2/hr = micromoles of oxygen per hour, t = time of dark storage in hours, I.B. = initial bottle, L.B. light bottle, and D.B. dark bottle, and 1 hour = time of incubation. The 1-hour incubations were conducted within the periods 8:30 a.m. to 10:30 a.m. and 1:00 p.m. to 4:00 p.m. on clear, calm days during June and July, 1974. Intensity of light was measured at 10-inch depth intervals with a Tsurumi-Seiki submarine illum- inance meter.- Measurements were repeated every 15 minutes during each incubation period. Approximately 93 percent of the light passed through one side of the BOD bottles, so all meas- ured light intensities were multiplied by a factor of 0.93. Competition Between Phytoplankton and Underwater Weeds The extent of coverage by aquatic weeds of some ponds on the Fisheries Research Unit and in the vicinity was estimated during June and July, 1972. Estimates of turbidity were by Secchi disk visibility.2 Phytoplankton density was estimated from chlorophyll a analysis (20) and from primary productivity deter- minations by the oxygen light-dark bottle technique (1). In two ponds, measurements of Secchi disk visibility, chlorophyll a con- tent of water, and cover of bottom by macrophytes were made at biweekly intervals from February 14 until August 31, 1973. Concentrations of soluble inorganic and total phosphorus in water samples were measured by standard techniques (1). To estimate standing crops of macrophytic algae, an open- ended plastic cylinder (10-inches in diameter) was pressed into the mud. Enclosed plant material was either hand-picked or re- moved with a dip net. All debris was removed and algae dried at 2200 Fahrenheit. Three to five, 10-inch diameter circles of algae were harvested from each pond. 1 An underwater light meter. 2The depth at which an 8-inch diameter disk with two black and two white quadrants disappeared from view. [5] Ecological Effects of Duckweeds Twelve circular pools (10 feet in diameter) were built in an unshaded area on the Fisheries Research Unit; they consisted of vinyl plastic liners attached to corrugated steel siding. The bot- tom of each pool was filled to 3-inch depth with a Cecil sandy loam top soil and water was maintained at depths of 20 to 24 inches by periodic additions or removals. On August 20, 1974, Spirodela polyrhiza and Wolffia columbiana were added to cover the surfaces of each of four pools, respectively. Four pools were maintained as controls. Fertilizer (3 ounces of a 20-20-5 mix- ture) was added to each pool on August 20, 1974, and again on September 10. Changes in water levels caused by evaporation and evapo- transpiration were measured daily from August 23 to September 25, 1974, with a hook gage. A rain gage was positioned beside pools for correction of data on water level changes during days when it rained. Maximum-minimum air temperatures were ob- tained daily from thermometers mounted in a Weather Bureau- type instrument shelter. Water temperature and dissolved oxygen concentrations were measured daily between 2:00 and 2:30 p.m. at 0, 3, 10, and 20-inch depths with a submersible thermister and a polarographic oxygen meter. Light penetration measurements were made near noon with a submersible photometer. Gross photosynthesis was measured on three dates by the oxygen light- dark bottle technique (1). Bottles were incubated at 10-inch depth. On September 20, water samples were collected from a depth of 10 inches for phytoplankton enumeration and chemical analysis (1). Standing crops of W. columbiana and S. polyrhiza were deter- mined on two dates. Plants were collected by placing a fine- mesh, 8-inch diameter sieve beneath the mats of duckweeds and then lifting the sieve. Fronds were removed from the sieve and dried at 220 ° F to constant weight. RESULTS AND DISCUSSION Effects of Light Intensity on Photosynthesis by Aquatic Weeds Relationships between light intensity and net photosynthesis in eight species of macrophytic algae and five species of sub- mersed higher aquatic plants are presented in figures 1 and 2. All regression equations were quadratic and correlations between [6] loo 1 Rhizoclonium hieroglyphicum 80 O/0 40 0, Hydrodictyon re ticula turn /O O/ 0 60 30 0. 0 - 0/0 '0 U) 0' / I / Chara vulgaris 40 / 20 F 0/ 0 Pithophora kewensis / 0 20 0 0 01 S Spiroyra sp 40 * Cladophora crispata o20 O" N 30 0 E5 20 - 0/ 0 o/ " to- 0 10 20 30 40 50 0 to 20 30 40 50 LUX X 103 LUX X 103 FIG.