1. Zooplankton use macrophytes as day-time refuge areas when trying to escape from pelagic predators. But macrophytes can also host a diverse and abundant macroinvertebrate assemblage and zooplankton are also likely to face predacious macroinvertebrates once they enter the littoral zone. This study aimed to elucidate the role of macroinvertebrates in determining the refuge capacity of macrophytes. 2. We conducted a field enclosure experiment using plastic bags and complementary laboratory feeding trials to test how macroinvertebrates counteract the benefits to zooplankton of the macrophyte refuge. The field experiment consisted of three treatments with different macroinvertebrate assemblages: without predators (WP), low abundance and diversity (LAD) and high abundance and diversity of predators (HAD -which represents lake conditions). 3. Populations of Diaphanosoma brachyurum, Bosmina huaronensis and Moina micrura (Cladocera) and of both male and female Notodiaptomus incompositus (Copepoda, Calanoida) declined (by nearly 80%) in the presence of HAD in comparison to WP and LAD treatments. 4. Feeding trials revealed that Buenoa sp. (backswimmer), adults of Palaemonetes argentinus (grass shrimp) and Cyanallagma interruptum (damselfly) had a significant negative impact on cladocerans (D. brachyurum, B. huaronensis) and the calanoid copepod population (males, females and copepodites). These predators showed a strong predation effect ranging from 75% to 100% reductions of zooplankton populations. 5. The refuge effect offered by macrophytes to zooplankton depends on and is balanced by the predacious macroinvertebrate assemblage that plants host. The risk of confronting littoral predators is high and macroinvertebrate presence can turn the macrophytes into risky areas for zooplankton.

SUMMARY:The female reproductive biology of Chasmagnathus granulatus Dana, a semiterrestrial burrowing crab endemic to the southwestern Atlantic, was compared in two contrasting coastal habitats: San Antonio (SA, marine) and Mar Chiquita (MC, estuarial). Mature females were collected monthly for 1.5 years and the ovarian cycle was described using a qualitative scale. Gonadosomatic (GSI) and hepatosomatic (HSI) indexes were calculated. The highest GSI and HSI occurred early in the reproductive season in SA and during the non-reproductive season in MC. The beginning and duration of the reproductive season also differed between populations: it started later and was shorter in SA. In MC, secondary vitellogenesis continued when the reproductive season had finished, and the ovaries remained fully developed throughout the nonreproductive season (winter). Therefore, females of MC were ready to lay eggs as soon as spring environmental conditions appeared. However, SA females did not attain a fully developed ovary during winter. A limited food supply would restrict the available energy to complete secondary vitellogenesis at the end of the reproductive season in SA, after the last spawning. Thus, the vitellogenic cycle should be completed in the following spring, causing a delay in the beginning of the reproductive period. In addition, the higher temperature amplitude may cause the reproductive period in SA to end early.Keywords: estuary, crab, Argentina, ovarian cycle, gonadosomatic index, hepatosomatic index. RESUMEN: DIFERENCIAS INTERPOBLACIONALES EN EL CICLO REPRODUCTIVO FEMENINO DEL CANGREJO ESTUÁRICOCHASMAGNATHUS GRANULATUS DANA, 1851 (BRACHYURA: GRAPSOIDEA: VARUNIDAE) DEL ATLÁNTICO SUDOCCIDENTAL. -La biología reproductiva de las hembras de Chasmagnathus granulatus Dana, un cangrejo semiterrestre y excavador endémico del Atlántico Sudoccidental, fue comparada entre dos localidades costeras con regímenes ambientales contrastantes: San Antonio (SA, marino) y Mar Chiquita (MC, estuarial). Hembras maduras fueron recolectadas mensualmente por 1.5 años y el ciclo ovárico fue descrito usando una escala cualitativa. Los índices gonadosomático (GSI) y hepatosomático (HSI) fueron calculados. Los mayores valores de GSI y HSI tuvieron lugar al inicio de la estación reproductiva en SA, y durante la estación no reproductiva en MC. El inicio y duración de la estación reproductiva también difirieron entre las poblaciones: ésta comenzó más tarde y fue más corta en SA. En MC, la vitelogénesis secundaria continuó cuando la estación reproductiva había finalizado, y el ovario permaneció desarrollado durante la estación no reproductiva (invierno). En consecuencia, las hembras de MC estaban listas para poner sus huevos tan pronto como aparecieron las condiciones ambientales de primavera. Por el contrario, las hembras de SA no alcanzaron un completo desarrollo del ovario durante el invierno. Un suministro limitado de alimento restringiría la energía disponible para completar la vitelogénesis secundaria al final de la estación reproductiva e...

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