Odonates in ponds of agricultutal areas: The influence of water quality?

Background/Question/Methods: Wetlands located in the province of Valladolid (central Spain) are characterized by small, isolated ponds and vernal pools. This area has a Mediterranean climate with rainfall that creates high seasonal and inter-annual fluctuations in pond water levels. These wetlands possess high biological diversity and contribute to important ecosystem services. However, they are fragile and difficult to maintain particularly when impacted by nearby farming activities. A better understanding of pond ecosystem dynamics is required to prevent degradation and loss of their biological wealth. The first phase in our investigation was to ascertain the status of representative ponds located in agricultural areas.

We accomplished this by analyzing water quality and identifying the community of Odonata insects inhabiting seven different ponds. Three ponds were protected from agricultural activities: Site of Community Interest (SCI), Coca and Olmedo, while four other ponds were heavily impacted. During August, pH, conductivity, and salinity of water in each pond were measured. Between May and October 2011, adult Odonata were sampled during thirteen visits.

Results/Conclusions:  The following ranges of values were obtained in these ponds: pH 8.4-9.7; conductivity 1023 mS cm^-1-3892 mS cm^-1; salinity 0.69 mg l^-1-3.14 mg l^-1. High values were positively associated with human activity such as over grazing and excessive fertilizer use.

Overall, twenty-three species of Odonata were observed (12 Zygoptera and 11 Anisoptera). Only three species, Lestes barbarus, Ischnura graellsii and Anax imperator, were present in all ponds. One species, Coenagrion caerulescens, is listed as Vulnerable on Spain’s Red List for the Order Odonata. Another species, Aeshna affinis, is considered rare on the Iberian Peninsula. Five ponds had similar numbers of species (17-23); two ponds had low numbers (3 and 6). Low diversity ponds had the poorest water quality or were ephemeral. These conditions made it difficult for Odonata to complete their life cycle.

Further studies must be made to determine the threshold of pollution and water source reliability that are required to sustain Odonata and healthy ponds.