Comparison of Biomass, Biodiversity, and Physical Characteristics between a Wetland Affected by Rice Agriculture and a Tidal Wetland in Palo Verde National Park, Costa Rica.

Category: Group Project

Participants: Patricia Argetsinger, Shannon Bunch, Rick Dutra, Melanie Phelps, Carol Richlin, Kermit Simons.

Site: Poza Verde Wetland and Quebrada Mula Wetland in Palo Verde National Park, Costa Rica.

Keyword: Blackwater Wetland, Restricted Random Study Site

Summary: We compared the biomass and biodiversity of two wetlands, one that drained a rice field and is affected by fertilizers and pesticides and one that is a tidal marsh. We found that the wetland by the rice field had a greater biomass but a much less biodiversity than the tidal marsh.

Introduction:

We studied two wetlands to compare the biodiversity, biomass, and the physical characteristics of each site. Site A, Poza Verde Wetland, borders the boundaries of the Palo Verde National Park. The site receives runoff water from adjacent commercial rice fields that are treated with fertilizers and pesticides, which are used extensively to increase production of rice in the Guanacaste Providence where the study site is located. (Janzen 1983.) The wetlands previously had been used for cattle grazing. In 1975, the Costa Rican government acquired the Comelco ranch, included Palo Verde. (Janzen. 1983). A 1973 Costa Rica Geographic Survey’s, 1:50,000 topographic map shows the wetland covered 7 square kilometers. Due to rice field irrigation, the wetland now measures approximately 13 –14 square kilometers. An irrigation canal runs to the west of our study site and dissects the wetland area. Sparse tree growth was observed along the banks of the canal.

Site B, Quebrada Mula, is located in the eastern section of the Palo Verde National Park. It is a tidal wetland fed by the Tempisque River, with no run off from agriculturally impacted areas. The tidal wetlands cover thousands of square kilometers and is a blackwater marsh. Blackwater contains almost no sediment but of clear but very darkly colored due to dissolved plant organic material. (Kricher. 1997). We believe that there will be a greater biomass in the impacted wetland but a greater biodiversity in the tidal wetland. We believe that physical characteristics will be approximately the same at both sites, except the pH. We think the pH of the rice field wetland will be more acidic due to agricultural chemicals used in rice production.
 

Methods:

We studied the two wetlands on 26 July, 1999 during the wet season. At each study area, we selected three restricted random study sites, meaning that we randomly selected sites while trying to include the major vegetation types of the area. Each study site included 2 square meters, which we marked off with string. We cut all vegetation within each plot just above the roots, bagged the vegetation and carried it to a clearing, where we sorted it by species. We air dried the plants for a minimum of fifteen minutes, then weighed them by placing them in a bucket on the gram scale to determine biomass. The scale had been calibrated not to calculate the weight of the bucket. We took the mass of several specimens of each species to calculate the average mass per plant and then using total biomass figures, approximated the number of plants per species in our samples.

We collected physical data at each site, which included air and water temperature, pH, turbidity of standing water, current, water depth, and a summary analysis of the soil composition. We noted the human/environmental interaction and noted the distance from site to human features.
 

Results:

We determined that the total biomass at Poza Verde was 29.954 kilograms. The total biomass at Quebrada Mula was 23.544 kilograms. The biomass was higher in the area that received fertilizer and pesticide runoff. The biodiversity was significantly greater in Quebrada Mula, the natural wetland. Using the Shannon-Wiener Index, we calculated the biodiversity to be .1657 for Poza Verde, and .5420 for Quebrada Mula. See Fig. 1. We found five plant species in the impacted wetland and 16 species in the tidal wetland. See Fig. 2. Water and air temperature were approximately equal. Turbidity was greater at Poza Verde, probably due to increased erosion for the rice fields. The pH at Poza Verde averaged 6.2, while pH at Quebrada Mula averaged 5.6. Soil samples at both sites were predominately silt and decaying vegetation.
 

Discussion:

Our results confirmed our hypothesis regarding both biodiversity and biomass. The greater biomass found in Poza Verde may be a result of fertilizer runoff from the rice paddies. (Janzen, 1983 ). Excess nutrients may be provided to plants in this area.

Further studies that may be conducted may include the following: Does the use of fertilizer in the rice paddies impact the biodiversity found in Poza Verde? If fertilizer concentrations are specific to a particular species of plant, could the growth of certain plants be accelerated at the expense of the others?
 
 

Works Cited:

1. Janzen, Daniel H. Costa Rican Natural History, University of Chicago Press, Chicago. pp. 105-106, 117.
2. Kricher, John. 1997 A Neotropical Companion. Princeton University Press, Princeton, NJ. pp 17.