Aquatic insect diversity and abundance in relation to dissolved oxygen content in streams in the TilarÃ¡n mountain range Susan Boehm Department of Ecology and Evolutionary Biology, University of Connecticut Abstract Aquatic invertebrates are affected by biotic as well as abiotic factors. Dissolved oxygen is one factor of water quality that has the potential to limit aquatic insect diversity in streams. Insects in the order Odonata ing that organisms in this order can only survive in healthy, undisturbed streams. This study compared dissolved oxygen levels and species richness as well as odonate larva abundance in ten sample sites in streams in the TilarÃ¡n mountain range. Trichopter a and Ephemeroptera were the most abundant orders in all sites and showed no relationship with dissolved oxygen level. However, the number of species was significantly positively related to dissolved oxygen. Odonata larval abundance peaked at a dissolved oxygen level of 10.3 mg/L and then decreased at higher levels. This drop off in abundance could be due to a change in substrate size, varied speed of water flow or too much turbidity in the sample site. More odonate larvae were found in streams where ther e was more adult activity. Relating species diversity and abundance to abiotic factors in streams is important for determining what effects pollution and disturbance are having on aquatic habitats. Resumen Los invertebrados acuÃ¡ticos son afectados por factores tanto biÃ³ticos como abiÃ³ticos. El oxÃgeno disuelto es uno de los factores de la calidad del agua que tiene el potencial de limitar la diversidad de insectos acuÃ¡ticos en quebradas. Se sabe que los insectos en el orden Odonata pueden ser utilizados como especies indicadoras, lo que significa que estos organismos pueden sobrevivir solamente en quebradas limpias y prÃstinas. Este estudio comparÃ³ las concentraciones de oxÃgeno disuelto y la riqueza de las especies, asÃ como la abundancia de las larvas de odonatos en diez sitios de muestreo en quebradas en la Cordillera de TilarÃ¡n. Los Ã³rdenes Trichoptera y Ephemeroptera fueron los mÃ¡s abundantes en todos los sitios y no mostraron ninguna relaciÃ³n con la concentraciÃ³n de oxÃgeno disuelto. Sin embargo, e l nÃºmero de especies estuvo positivamente relacionado con el oxÃgeno disuelto. La abundancia de larvas de odonatos alcanzÃ³ su mÃ¡ximo a una concentraciÃ³n de oxÃgeno disuelto de 10.3 mg/l y despuÃ©s disminuyÃ³ en niveles mÃ¡s altos. Esta disminuciÃ³n en abundan cia podrÃa haber sido causada por un cambio en el diÃ¡metro del sustrato, la velocidad cambiante de la corriente de agua, o debido a la turbiedad excesiva en el sitio. Se encontraron mÃ¡s larvas de odonatos en quebradas donde hubo mÃ¡s actividad de los adulto s. Es importante relacionar la diversidad y la abundancia de las especies con los factores abiÃ³ticos en las quebradas para determinar cuÃ¡les efectos son causados por la contaminaciÃ³n y la pertubaciÃ³n de los hÃ¡bitats acuÃ¡ticos. Introduction Dragonflies an d damselflies, each belonging to the order Odonata and suborders Anisoptera and Zygoptera, respectively, are well known biological indicators of water quality and stream health. Taxa are labeled bio indicators based on their sensitivity to certain environ mental attributes in this case, dissolved oxygen content in streams (Foote and Hornung 2005). Odonata fill an important role in stream ecosystems and food webs. Adult as well as larval odonates are predacious and important in controlling other insect po pulations such as mosquitoes and other possible pests (Lehmkuhl 1979). The life cycles of odonates always begin in water. Females deposit eggs in streams using an
ovipositor and it is there that the larvae develop. These larvae, or nymphs, go through se veral molts before emerging as an adult usually during dryer weather (Daly et al. 1998). Aquatic insects require sufficient amounts of oxygen to survive. The ability for water to hold dissolved oxygen is negatively correlated with temperature. Since s tream temperature can change daily and seasonally the concentration of dissolved oxygen also fluctuates. Larger streams and rivers are less variable in regards to temperature because of the high flow rate, whereas unshaded and smaller streams experience h igher daily fluctuations (Allan 1995). Many aquatic insects are unable to move water through their gills on their own and thus require running water. This means that a lower oxygen level may be tolerated in faster running but not in slow moving currents (Allan 1995). Previous research has indicated a preference in aquatic macro invertebrate organisms to inhabit mid sized to large substrate pebble or cobble (Hyman 2002). Substrate size in streams is determined by speed of the water flow. Slower flowin g water will allow finer sediment to settle while fast flowing water will sort out larger particles. Lithophilous taxa refer to organisms found inhabiting stony substrate (Allan 1995). This was the type of insect taxa examined in this study. Slower curr ents are known to be correlated with finer substrate particles but also possibly lower oxygen levels (Allan 1995). This study looked at the relationship between oxygen levels and aquatic insect diversity with special attention given to odonates. A recen t study found a positive correlation between overall aquatic macro invertebrate diversity and abundance with the diversity and abundance of odonate larvae (Foote and Hornung 2005). It was hypothesized that the sampling sites with higher dissolved oxygen l evels would contain the highest diversity of aquatic insects. Also, odonate larvae were predicted to be more common in healthier streams with more dissolved oxygen. Methods Study Sites Ten sampling locations were chosen: 8 in the Monteverde region an d two located in RÃo CaÃ±itas just beyond El Dos de TilarÃ¡n (Figure 1). The streams sampled in Monteverde consisted of two tributaries of RÃo Lagarto, Quebrada MÃ¡quina, RÃo Negro, and Quebrada Moras. Sampling Sampling took place July 17 th August 5 th a t various times during the day. At each site a 15 meter transect along the stream was chosen based on accessibility and presence of pebble to large cobble sized substrate. Along each transect, four samples were taken evenly spaced within the 15 meters. A total of 40 samples were collected. A kick net was used for collection. Standing upstream from the net, substrate was disturbed by kicking for one minute within 1 meter of the net. Suspended particles and potential organisms in the water were allowed to settle out for 30 seconds before removing the net. This was usually more than enough time for the water to become clear again. Organisms were then sorted out from the leaf litter and sediment in the net and preserved in small glass vials of diluted a lcohol. A small container or pan was helpful for sorting out organisms. Each of the subsequent three samples was taken upstream from the previous one. If a sample site was unsuitable, for example if a large boulder was present or the water was too deep t o walk through, then the sample was taken at the closest reasonable
site while trying to maintain spacing within the transect. A dissolved oxygen meter was used to determine the concentration of dissolved oxygen (mg/l) and temperature at each sample site. Qualitative notes were made on the quality of substrate, relative speed of flow and size of stream, and openness or shadiness of the site. Presence and general activity of adult Odonata was also recorded. Organisms were sorted and identified using a di ssecting scope. Identification was organized by order of the organism and then separated into morphospecies within the orders. Any organism caught that was not an insect was noted but discarded in the field. Analysis Regressions were used to determin e if dissolved oxygen levels were significantly correlated with the number of individuals in each sample or species richness in each sample. Results Out of all 40 samples from the 10 sites, 16 morphospecies of aquatic insects were identified representi ng 7 orders. All sites, regardless of location or dissolved oxygen were dominated by the presence of Trichoptera larvae (Table 1). The three most abundant orders, Trichoptera, Ephemeroptera, and Plecoptera, did not show any correlation with level of dissol ved oxygen. Dissolved oxygen was found to positively affect the number of species in each sample (simple regression, R 2 = 0.155, p = 0.0118) (Figure 2). However, the number of individuals in each sample did not increase as strongly with higher dissolved o xygen (simple regression, R 2 = 0.086, p = 0.0663) (Figure 3). Aquatic insect diversity was significantly correlated with the average dissolved oxygen level at each site (Figure 4). A significant negative correlation was found between the diversity at each site and temperature (Figure 5). Odonata nymphs were found at 6 of the 10 sites their abundance was found to increase when dissolved oxygen levels were greater than 9.5 mg/L and then peak at 10.3 mg/L after which they declined (Figure 6). Based on observa tions of the openness or shadiness of the sites, Odonata were more likely found in open and sunny areas of wider streams than closed in shady areas with lots of vegetation very close to the water. Adult Odonata activity was observed at all but one site where larvae were found (Table 2). Activities included mating, defense of territories and flying. Discussion Due to the extreme abundance and presence at all but one site (Site 4 where no organisms were found in any sample) it appears that Trichopte ra larvae and Ephemeroptera morphs are generalists in regards to water quality. According to pollution tolerance scores (Lehmkuhl 1979) almost all families within the order Ephemeroptera can tolerate higher levels of pollution and lower oxygen levels. Th e same is true for Trichoptera so it is not unusual that these orders were found in the highest numbers and in the most sample sites. The presence of more odonates in waters with relatively high dissolved oxygen supports the initial hypothesis. Few famil ies within Odonata can tolerate poor water quality (Lehmkuhl 1979) and no odonate larvae were found in sites that were closest to houses and farms where pollution enters the streams.
Opposing trends were found in relating diversity at each site to tempe rature and therefore probably cannot support as many aquatic life forms. Diversity was lower in samples that had a higher temperature and lower dissolved oxygen level. Aquatic insects cannot tolerate very low oxygen levels which often occur in polluted streams in disturbed areas. Thus, the healthiest streams tend to have a higher diversity of aquatic insects, appears then, that among the streams that were sampled, RÃo Negro and Quebrada MÃ¡quina are the least disturbed and those sites were in or near continuous forest. While the highest recorded dissolved oxygen level was 10.8 mg/l the highest abundance of Odon ata larvae was found at 10.3 mg/l. This suggests that oxygen levels increased beyond a certain point may not result in a higher abundance of odonates, which is a trend that is not described in any of the cited literature. This apparent drop off in abunda nce may have occurred because water with very high dissolved oxygen content also has high turbulence. Small aquatic insects may be less likely to inhabit water with too much turbulence because their body forms cannot support the stress of being pushed aro und in the stream and it is difficult to attach to the substrate (Allan 1995). The decrease in abundance may also be due to slightly different substrate size, which is determined by speed of water flow. The trend of fewer individuals after a certain level of dissolved oxygen appears to be true for all the morphospecies found (Figure 3). While a past study did not find any significant correlation between adult odonate behavior and the presence of larvae, this study indicates that a relationship exists. Th e highest abundance of odonata larvae was in sites where territory defending behavior was observed. Adult odonates are more active in bright sunlight and this may explain why more were seen around wider and less shaded streams. The presence and type of a dult activity also appears to be correlated with better sites to find larvae. Since it is the preferred habitat for the adults, more females will be laying their eggs in sunnier and more open areas with high water quality, leading to a higher larval abund ance. Acknowledgements I would like to thank my parents for using up their frequent flyer miles to buy me a ticket to Costa Rica. I Guindon for bein g so understanding and helpful and driving me around on his awesome motorcycle. Aloha nui loa e mahalo nui to Javier MÃ©ndez for, well, just being Javier. Muchas gracias to all the Ticos I met during my sampling adventures: the guy that let me carve a wood en bowl and eat bananas on his farm, the many relatives of Mireya in TilarÃ¡n, MÃ³nica who shared her guayabas with me, and Charlie who makes artwork out of scrap metal.
Literature Cited Allan, J. David. 1995. Stream ecology: structure and function of running waters. Chapman and Hall, New York.pp.23 81. Daly, H. V., J. T. Doyen, and A. H. Purcell III. 1998. Introduction to insect biology and diversity. Oxford University Press, New York. Foote, A. Lee, C. L. Rice Hornung. 2005. O donates as biological indicators of grazing effects on prairie wetlands. Ecological Entomology. June, Volume 30. p. 273 Hyman, O. 2002. Substrate effects on macro invertebrate composition and guild structure. CIEE Monteverde, Fall. pp. 86 100 Lehmk uhl, D. M. 1979. Aquatic insects. Wm. C. Brown Company Publishers, Dubuque, Iowa. pp. 74 85 Steach, T. 2004. Damselfly behavior in a tropical cloud forest. CIEE Monteverde, Spring. pp. 70 82
Table 1. Li st of all unique morphospecies of aquatic insects identified in descending order of total number of individuals of each morph in all 40 samples from 10 stream sites in the TilarÃ¡n mountain range. Order (Morph) Total Abundance in All Sample s Trichoptera 144 Ephemeroptera (Morph 1) 30 Plecoptera 26 Ephemeroptera (Morph 2) 22 Ephemeroptera (Morph 3) 16 Hemiptera (Morph 1) 13 Odonata Zygoptera 8 Coleoptera (Morph 1) 5 Hemiptera (Morph 2) 5 Odonata Anisoptera 4 Coleoptera (Morph 2) 3 Unknown adult insect sp. 2 Coleoptera (Morph 3) 1 Hemiptera (Morph 3) 1 Hemiptera (Morph 4) 1 Diptera 1 Table 2. Adult Odonata behavior and presence of odonate larvae at 10 stream sites in the TilarÃ¡n mountain range. S ee Figure 1 for location of sites. Mean + 1 S.D. also given for dissolved oxygen (mg/L) and temperature ( 0 C). Site Odonata Larva Present D.O. (mg/L) Temp ( 0 C) Adult Odonata Behavior 1 Yes 10.3 15.5 Two individuals mat ing 2 No 9.6 20.7 None 3 No 9.8 20.3 None 4 No 9.6 17.5 None 5 Yes 10.3 17.6 None 6 Yes 10.2 16.6 Multiple pairs mating and defending territory 7 Yes 10.3 14.9 Several individuals flying 8 No 10.1 15.5 None 9 Yes 10.4 15.8 One individual flying 10 Yes 10.1 15.9 One individual defending territory
Figure 1. Topographic map of the TilarÃ¡n mountain range showing 10 stream sites. 1 square is equal to 1 km 2 Figure 2. Relationship between dissolved oxygen and the number of morphospecies of aquatic insects present in 40 samples from 10 stream sites in the TilarÃ¡n mountain range. (R 2 = 0.155, p = 0.012) D issolved Oxygen (mg/L)
Figure 3. Rela tionship between dissolved oxygen and the number of individuals of aquatic insects found in 40 samples from 10 stream sites in the TilarÃ¡n mountain range. (R 2 = 0.086, p = 0.066) Figure 4. Relationship between Shannon We mean dissolved oxygen level for 10 stream sites in the TilarÃ¡n mountain range. Dissolved oxygen mg/L
Figure 5. Relationship between Shannon Weiner diversity index as a function of mean temperature of 10 stream sites in the TilarÃ¡n mountain range. Figure 6. Relationship between abundance of Odonata larvae as a function of dissolved oxygen level in 40 samples in the TilarÃ¡n mountain range.
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Diversidad y abundancia de insectos acuticos en relacin con el contenido de oxgeno disuelto en los ros de la Cordillera de Tilarn
Aquatic insect diversity and abundance in relation to dissolved oxygen content in streams in the Tilarn mountain range
Aquatic invertebrates are affected by biotic as well as abiotic factors. Dissolved oxygen is one factor of water quality that has the potential to limit aquatic insect diversity in streams. Insects in the order Odonata
in particular have been known to be indicator species meaning that organisms in this order can only survive in healthy, undisturbed streams. This study compared dissolved oxygen levels and species richness
as well as odonate larva abundance in ten sample sites in streams in the Tilarn mountain range. Trichoptera and Ephemeroptera were the most abundant orders in all sites and showed no relationship with
dissolved oxygen level. However, the number of species was significantly positively related to dissolved oxygen. Odonata larval abundance peaked at a dissolved oxygen level of 10.3 mg/L and then decreased at higher levels. This drop off in abundance could be due to a change in substrate size, varied speed of water flow or too much turbidity in the sample site. More odonate larvae were found in streams where there was more adult activity. Relating species diversity and abundance to abiotic factors in streams is important for determining what effects pollution and disturbance are having on aquatic habitats.
Los invertebrados acuticos son afectados por factores tanto biticos como abiticos. El oxgeno disuelto es uno de los factores de la calidad del agua que tiene el potencial de limitar la diversidad de insectos acuticos en quebradas. Se sabe que los insectos en el orden Odonata pueden ser utilizados como especies indicadoras, lo que significa que estos organismos pueden sobrevivir solamente en quebradas limpias y prstinas. Este estudio compar las concentraciones de oxgeno disuelto y la riqueza de las especies, as como la abundancia de las larvas de odonatos en diez sitios de muestreo en quebradas en la Cordillera de Tilarn. Los rdenes Trichoptera y Ephemeroptera fueron los ms abundantes en todos los sitios y no mostraron ninguna relacin con la concentracin de oxgeno disuelto. Sin embargo, el nmero de especies estuvo positivamente relacionado con el oxgeno disuelto. La abundancia de larvas de odonatos alcanz su mximo a una concentracin de oxgeno disuelto de 10.3 mg/l y despus disminuy en niveles ms altos. Esta disminucin en abundancia podra haber sido causada por un cambio en el dimetro del sustrato, la velocidad cambiante de la corriente de agua, o debido a la turbiedad excesiva en el sitio. Se encontraron ms larvas de odonatos en quebradas donde hubo ms actividad de los adultos. Es importante relacionar la diversidad y la abundancia de las especies con los factores abiticos en las quebradas para determinar cules efectos son causados por la contaminacin y la perturbacin de los hbitats acuticos.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone
Diversidad de especies
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Summer 2005
Ecologia Tropical Verano 2005
t Monteverde Institute : Tropical Ecology