An Inquiry-Based Field & Laboratory Investigation of Leaf Decay: A Critical Aquatic Ecosystem Function

Effective investigations incorporate all four features of constructivist teaching. This high school or college-level field investigation guides teachers (and students) through the stages of inquiry. The focal concept is ecosystem function, specifically leaf decay rates in aquatic environments. Teachers elicit their students' prior knowledge and use it to generate discussion on variables that influence decay rates. Students engage in designing and conducting experiments. The learning cycle is continued when students apply their new knowledge and receive feedback, and completed when students return to their initial conceptions of leaf decay and reflect on the knowledge they gained through scientific experimentation.

Linking shrimp assemblages with rates of detrital processing along an elevational gradient in a tropical stream

We experimentally excluded freshwater shrimp assemblages (Atyidae, Xiphocarididae, and Palaemonidae) to examine their effects on detrital processing and benthic insect biomass at three sites along an elevational gradient in a tropical stream in Puerto Rico. We also determined which shrimp taxon was responsible for leaf decay in a subsequent laboratory experiment. At the high-elevation site, the shrimp assemblage was dominated by Atya spp. and Xiphocaris elongata, and leaf decay rates were significantly faster in the presence of shrimps than in their absence. Laboratory experiments showed that this was primarily due to direct consumption of leaves by Xiphocaris. Shrimps had no effect on leaf decay rates at mid- and low-elevation sites where there were higher proportions of Macrobrachium spp. shrimps (which prey on Xiphocaris). Laboratory experiments showed that Xiphocaris consumed significantly less leaf material and experienced significantly higher mortality in the presence of Macrobrachium. Shrimp exclusion resulted in significantly less and significantly more insect biomass at the high- and low-elevation sites, respectively; no difference was found at the mid-elevation site. Insects played a minor role in leaf decay. Results show a strong linkage between shrimp assemblages and rates of detrital decay and illustrate the importance of conducting experiments at multiple sites.

Leaf Blight of Endive and Escarole, Caused by Rhizoctonia solani, in California

Endive (Cichorium endivia) and escarole (broad-leaf type of C. endivia) are two of the many leafy vegetables produced commercially in coastal California. For several years, both crops have been affected by a disease that causes soft, watery, brown leaf decay. In the tightly appressed heads of endive and escarole plants, decay generally spreads in a concentric circle, resulting in circular whorls of brown, rotted leaves within diseased heads. Such symptoms make the heads unmarketable. In the Salinas Valley (Monterey County), the disease was much more prevalent during 1998, when weather was affected by “El Niño.” Rhizoctonia solani was isolated consistently from symptomatic leaves of both endive and escarole. Pathogenicity was tested by placing two agar plugs of representative isolates inside the leaf whorls of 15 potted plants each of endive (cv. Tres Fine Maraicchere) and escarole (cv. Full Heart). Watery, brown leaf decay, similar to symptoms observed in the field, occurred on all plants within 7 days after inoculation, and R. solani was reisolated. Control plants, treated with sterile agar plugs, did not develop disease. Tests were repeated, and results were similar. Anastomosis-group testing revealed that four endive isolates belonged to AG2-2 (1). This appears to be the first report of leaf blight of endive and escarole caused by R. solani in California. Reference: (1) Sneh et al. 1991. Identification of Rhizoctonia Species. The American Phytopathological Society, St. Paul, MN.

Relative Importance of Microbes versus Macroinvertebrate Shredders in the Process of Leaf Decay in Lakes of Differing pH

Four nonhumic lakes in northern Michigan, ranging in pH from 4.0 to 8.0, were selected to assess the effects of pH on leaf decay rates, leaf-associated macroinvertebrate assemblages, and the relative importance of microbes and shredders to the leaf decay process. Except for pH (and covariates of pH) these lakes were similar in physical and chemical parameters which directly affect metabolism. Preweighed leaves were placed in all four lakes for 8 wk; half of the leaves were confined in mesh bags to exclude shredders, while the others were not confined. Decay rates of confined leaves were not different among lakes, yet were lower than non-confined leaves in the alkaline lakes, suggesting shredders were important vectors of leaf decay in the alkaline lakes, but not in the acid lakes. Shredders comprised 65.9% of all invertebrates in Douglas Lake (pH = 8.0) and decreased in abundance (to 1.1%) with decreasing pH. Crustaceans and molluscs were few or absent in the acid lakes probably because of low pH and Ca2+ levels. Although microbial biomass on leaves appeared to be highest in acid lakes, estimates of microbial activity were significantly lower in the acid lakes, suggesting microbial metabolic inhibition at low pH.