Introduction

Floods result from rainfall events and shows effects on the biotic component rivers as macrophytes and invertebrates (^{Robinson & Uehlinger, 2003}; ^{Downes & Street, 2005}). Abundance of invertebrates can be reduced by 70-95% (^{Brooks & Boulton, 1991}; ^{Giller et al., 1991}). Invertebrates as Odonata actively move river margins during floods (^{Scrimgeour et al., 1988}; ^{Rempel et al., 1999}).

Macrophytes are negatively impacted by precipitation (^{Westwood et al., 2006}). Presence of macrophytes is generally related to the characteristics of the resistance and resilience of aquatic plants to hydrologic disturbances of flood and drought (^{Pedro et al., 2006}). Consequently, macroinvertebrates living near macrophytes as Odonata larvae are directly affected by hydrologic regimes (^{Corbet, 1999}; ^{Hofmann & Mason, 2005}).

Odonate larvae occupy a high variety of freshwater environments such as rivers and lakes (^{Merritt & Cummings, 1996}; ^{Pires et al., 2013}). Litoral zone is the site where Odonate larvae are registered more frequently, mainly close to macrophytes such as Salvinia auriculata Aubl. Odonata larvae use macrophytes as shelter against their main amphibian and fish predators. In addition, aquatic plants offer a high abundance of food resources to Odonate larvae such as oligochaetes, chironomid, mayflies, tadpoles and other preys.

It is evident that precipitation negatively affects macrophytes such as S. auriculata decreasing their abundance. This macrophyte in turn provides shelter for Odonata which can also be affected by precipitation. The goal of this work was to know the effect of precipitation in four periods (Rising, Flood, Lowing and Dry) on Odonata larvae living near to the macrophyte S. ariculata in Been River, Amazonas, Brazil. The Been river was chosen due to the presence of the macrophyte studied throughout the year allowing the study.

Material and methods

The biological material was collected in the Been river in the section near the river Madeira, state of Amazonas, Brazil (Fig. 1). Sampling was conducted in stands of three macrophytes S. auriculata in four periods: rising (November 2012), flood (February 2013), lowing (May 2013), and dry (August 2013) in the site (7^o 32’ 39” S; 63^o 01’ 09” W). All samples were taken with hand sieves (mesh = 0.5 mm) carefully placed below the macrophytes. The specimens collected were fixed and preserved in 70% ethyl alcohol. Odonata larvae were identified to genus level, using specialized keys (^{Carvalho & Calil, 2000}; ^{Costa et al., 2004}; ^{Neiss, 2012}).

Figure 1 Rio Been near the river Madeira, state of Amazonas, Brazil. 

Canonical Correspondence Analysis CCA was performed in the PAST 3.25 (free software). Canonical Correspondence Analysis was used to analyze the relationship between species abundance and periods: rising (November 2012), flood (February 2013), lowing (May 2013), and dry (August 2013). The level of significance (5%) was established.

Results

A total of 286 Odonata larvae were identified in the river studied. Six families and 14 Odonata genera were identified (Table 1). The flood period showed the highest abundance in the studied period (336 ind.m^-2). The lowing period presented the lowest abundance (171 ind.m^-2).

The first two axes of the CCA explained 87.8% of the total variation (axis 1= 51.9%; axis 2= 35.9%). The CCA showed that Acanthagrion and Telebasis. Acanthagrion presented a positive correlation with flood period and Telebasis also positive however dry period.

Discussion

In this study, only two genera of Odonata, Acanthagrion and Telebasis showed a significant difference between their densities and the studied periods: rising, flood, lowing, and dry. Acanthagrion and Telebasis are suborder Coenagrionidae (Zygoptera). Coenagrionidae is characterized by small animals of 20 to 45 mm and short body and abdomen. Generally, the males show more intense colors than the females and they are distinguished of the other families for presenting a pigmented spot near the end of the wings (^{Lencioni, 2006}). Coenagrionidae is the family with the greatest number of genera and species in the Neotropical region (^{Kalkman et al., 2008}). In the Been river Coenagrionidae was the family that presented the greater number of genera (six).

The principal component analysis showed that Telebasis was positively correlated with dry period. A higher abundance of Telebasis in the dry period was previously recorded in the southeastern region of Brazil in floating macrophytes (^{Fulan & Henry, 2007}). Unfortunately, studies on fauna associated with macrophytes are not very frequent in Brazil. However, it is possible that there is some correlation between Telebasis and the dry period, because in two different studies and in different regions of Brazil (North and Southeast) the result was similar. ^{Robinson & Frye (1986)} observed in a Telebasis species a behavior of females to lay their eggs inside the stem of floating macrophytes. It is possible that a similar survival strategy could favor the increase of the Telebasis abundance in the months of lower precipitation, i.e., the dry period.

Unlike Telebasis, Acanthagrion was positively correlated with flood period. Acanthagrion currently has 39 species distributed in the southern United States to central Argentina (^{Garrison et al., 2010}). Species are small to medium size and are characterized by wings generally hyaline (^{Lozano & Anjos-Santos, 2012}). ^{Corbet (1999)} pointed out that few species have the ability to fly during the rainy season. However, according to author, some species of Acanthagrion showed high capacity to fly even during the rain. In Suriname these species are known locally as "rain dragonfly". In Brazil, Oxystigma petiolatum also have the ability to fly during the rain. It is possible that behavioral characteristics such as flying ability in the rainy season may have resulted in a higher abundance of Acanthagrion larvae mainly in the flood period.

Conclusion

In conclusion, although there are few studies on Odonata larvae associated with floating macrophytes in the Amazon, the results of this work showed the importance of knowing the distribution of larval abundance throughout the year. Flood and dry period showed to be the period with the highest abundance of Odonata genera. In addition, the CCA showed that some species are more sensitive to the periods of the year, Telebasis in the dry period and Acanthagrion in the flood period. The results of this work are only preliminary and nonconclusive, additional and more detailed studies are necessary mainly identifying the species that are associated with the macrophytes and not only the genus. However, this work may encourage new researchers in the search for knowledge of the Amazonian Odonata species.