Comparison of macroinvertebrate communities of intermittent and perennial streams in the dry forest of Guanacaste, Costa Rica Suzanne M. de Szoeke, 1,2 Thomas L. Crisman 3 * and Paul E. Thurman 3 1 Department of Environmental Engineering Sciences, University of Florida, Gainesville, FL, 32611, USA 2 current: GSI Water Solutions, Inc, Corvallis, OR, 97333-4108, USA 3 Department of Integrative Biology, University of South Florida, 4202 E. Fowler Ave, SCA110, Tampa, FL, 33620-8100, USA ABSTRACT The 7 m-wide Canal Oeste passes over a series of first-order, intermittent streams on hillsides on its way to irrigate the lowlands of Guanacaste, Costa Rica. Streams passing under lined portions of the canal have maintained intermittent flow, while those under unlined portions of the canal have become perennial, thus providing a unique opportunity to examine the impact of drastically altered hydrology on macroinvertebrate communities of dry forest streams. Macroinvertebrates were studied from the end of the dry season through a wet season. Macroinvertebrate recolonization of intermittent streams at the beginning of the wet season was rapid, led by small bodied taxa such as chironomids and oligochaetes with short life cycles and/or great tolerance for low oxygen and spates. As additional, less tolerant taxa arrived, most likely aerially from adjacent perennial streams, the fauna of intermittent streams progressively became similar to that of perennial streams of the area within 3–4 months of rewetting. The great resilience of the macroinvertebrate community is clearly demonstrated by its ability to adjust with few modifications to a complete switch from intermittent to perennial hydroperiod. Copyright © 2015 John Wiley & Sons, Ltd. KEY WORDS tropical streams; canal impacts; altered hydrology; intermittent; perennial; macroinvertebrates; Costa Rica; dry tropical forests Received 28 December 2014; Revised 25 June 2015; Accepted 26 June 2015 INTRODUCTION Dry forests are currently one of the most threatened lowland forest ecosystems in the tropics (Janzen, 1988). These habitats tend to be semideciduous forests dominated by trees ranging between 20 and 30 m-tall shrubs with thorns or spines, woody vines and ground bromeliads (Holdridge, 1967). Most precipitation (approximately 95% annually) falls during a 7- month period between May and November, with a short period of reduced rainfall occurring between late June and mid- August referred to as the ‘El Veranillo de San Juan’ (Jimenez et al., 2001). Thus, low-order, tropical dry forest streams typically display intermittent flow associated with distinct wet and dry seasons. Depending on precipitation patterns, low- order streams may flow intermittently or continuously during the wet season and cease flow or dry out during the dry season. Historically, dry forests comprised approximately 42% of the global tropical vegetation (Murphy and Lugo, 1986). In Central America, these habitats once covered an area five times larger than Guatemala; however, by the mid-1980s, vast areas of relatively pristine dry forest in the region had disappeared with less than 1% receiving some manner of conservation (Janzen, 1986). Many areas of tropical dry forest in Costa Rica have been lost through degradation, fragmentation and elimination by being converted to crop and pasture lands (Janzen, 1986). The remaining dry forest is located primarily in the Tempisque River Basin in the northwestern province of Guanacaste (Jimenez et al., 2001). In response to the dramatic seasonal differences in precipitation and stream flow, the Costa Rican government initiated the Arenal-Tempisque Irrigation Project (PRAT) in 1980 to increase agricultural production in Guanacaste during the dry season and decrease vulnerability to drought. This large-scale irrigation project conveys water from Lake Arenal in the central highlands westward to the agricultural lowlands of Guanacaste (Pacific, 2002), which subsequently facilitated additional conversion of lowland forests and wetlands into rice and sugar cane fields. By 2001, PRAT supported the production of approximately one-half of the rice and sugar cane production of Costa Rica (Jimenez et al., 2001). In 2003, PRAT completed a 21-km extension (phase 3) of one of its principal canals, Canal Oeste, that has influenced stream hydrology in the dry forests through which it passes. The initial section of the extension is concrete lined and has *Correspondence to: Thomas L. Crisman, Department of Integrative Biology, University of South Florida, 4202 E. Fowler Ave, SCA110, Tampa, FL 33620-8100, USA. E-mail: tcrisman@usf.edu ECOHYDROLOGY Ecohydrol. 9, 659–672 (2016) Published online 17 September 2015 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1665 Copyright © 2015 John Wiley & Sons, Ltd.