Angela Dombrowski Algal Growth and Environmental Effects 19 April 2006p. 1The Effects Of Nutrients, Light and Insect Herbivory on Algal BiomassAngela DombrowskiAbstract Algae are an important source of primary productivity in stream ecosystems, but theeffect of nutrients on their growth, in relation to light limitation and herbivory is not well knownin lotic systems. To better understand these two effects and the links between them, I examinedthe response of algal growth to light availability, nutrient availability and herbivory, by the useof nutrient diffusing substrates (nitrogen, phosphorus, and nitrogen + phosphorus) within theSouth Fork Eel River, Mendocino Co., CA. The study was conducted within nine streams withvarying degrees of canopy cover. Of these nine streams, four were chosen to examine the effectof herbivorous insects on increased algal growth. Nutrient diffusing substrates were used toassess the effects of nutrients on algal biomass. To see if herbivores respond to the expectedincrease in algal biomass by increased consumption and presence, treatments allowing forherbivore inclusion were added. Chlorophyll-α (productivity) and ash-free dry mass (AFDM,biomass) data were analyzed via multiple regression. I concluded that nutrients, when alone andnot in conjunction with one another, showed to have either an increased (N) or decreased (P)effect on algal productivity and biomass. In a Two-Way ANOVA, herbivores were not shown toreduce organic biomass in streams, nor did the data show them to have a significant preferencetowards nitrogen treatments. This could mean that either algal abundance does not play a role ingrazer nutritional preferences, or that grazing of algae on nutrient treated substrates allowedother algae, not preferred by grazers, to colonize nutrient treated substrates.Angela Dombrowski Algae and Environmental Effects 19 April 2006p. 2IntroductionThere are many factors that affect trophic systems in streams and much has been studiedregarding the variability of community structures, in combination with differing environmentalaspects. Because algae are important in both primary productivity and nutrient cycling withinaquatic ecosystems, it is important to understand how the effects of nutrient limitation, light andherbivory on algal growth and how these factors vary within streams. Although much has beenstudied regarding algal reactions to environmental manipulation in marine and lentic (lakes,ponds) ecosystems, less attention has been paid to effects of nutrients, light and herbivory in loticecosystems (streams).Nutrient and light availability in stream systems has an affect on various stream populationsand processes such as algal growth, invertebrates and leaf litter decomposition (Elwood et. al.1981, Peterson et. al. 1985, Hill and Knight 1988). It has been suggested that ratios of light tonutrient availability, as well as, the total availability of nutrients within the water column mayhave an effect on decomposition and invertebrate growth (Elser et. al. 2000, Sterner and Elser1997). Phosphorus, nitrogen, and a phosphorus/nitrogen combination have been shown to limitalgal growth in streams (Elwood et. al. 1981, Fairchild et. al. 1985, Grimm and Fisher 1986,Tate 1990).Previous studies have shown that as light availability increases, so does algal biomass (Rierand Stevenson, 2002, Hill and Knight 1988). Presumably, as stream size increases, canopy coverwill decrease, and hence microbial biomass will increase (Hamilton et al 2001). In addition, insitu shading experiments, affecting light availability, have shown to decrease chlorophyll-αconcentrations of algae (Hill and Knight 1988, Findlay and Howe 1993). It has also been shownthat as stream size increase to ≥ 10 km2, algal production becomes increasingly important to allconsumer groups excluding shredders. In the case of streams smaller than ≤10 km2, consumergroups follow the River Continuum Concept of community consumption patterns (Vannote et.al. 1980, Finlay 2001). It is suggested that this is due to a decrease in probably due to a decreasein terrestrial allochthonous inputs in relation to stream width and increasing algal productiondue to light availability (Vannote et. al. 1980).The importance of algae for herbivorous invertebrates has been documented (Hill et. al.1995, Hill and Knight 1998, Lawrence and Gresens 2004), and various studies have examinedAngela Dombrowski Algae and Environmental Effects 19 April 2006p. 3the effects of nutrients in combination with herbivory (Hill and Knight 1988, Finlay 2001, Rierand Stevenson 2002 Urabe et. al. 2002). This study examines how nutrient limitations and lightavailability affect algal growth in streams and how this growth is affected by herbivores.This investigation is, in essence, an extension of previous research performed in the UCAngelo Reserve by Walter Hill and Allen Knight in 1988. The Hill & Knight studies wereconducted in two second-order streams and consisted of two parts. The first and most relevant tomy study, found that algal biomass and herbivorous insects were greater on substrates withincreased nutrient availability in the unshaded stream. Because increased algal biomass wasfound to be significantly greater in the shaded stream, when light could penetrate the canopy, it issuggested that light is a primary limiting factor in algal growth. The second study found thatcaddisfly larvae (Neophylax) biomass was inversely proportional to the reduction of algalbiomass and mayfly nymphs (Ameletus) were shown to be selective grazers. In order to more fully understand how nutrient and light limitation affects algal growthwithin the stream drainage area of the South Fork Eel River, I investigated how environmentaldifferences and nutrient manipulations affect algal biomass within 9 streams and whether thenumber of herbivores on nutrient rich substrates is significantly different from control substrates.The experiment concerning herbivores was conducted within 4 of the proposed 9 streams. Thismakes the study an expanded, in situ analysis of the Hill and Knight studies within the SouthFork Eel watershed area, allowing characterization of the environmental inputs that affectprimary and secondary production.The hypotheses are that there will be an overall increase in algal growth on pots that areequipped with augmented nutrient and
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