Evaluation of the ornithogenic influence on the trophic state of East Mediterranean wetland ecosystem using trend analysis

The Great Rift Valley portion of the East African–Eurasian Migratory Flyway is extremely important globally because of the numbers (>500 million) and diversity of seasonal traveling birds. The construction of the Agmon wetland (1.1km2) in the Hula Valley, Israel in 1994 and a change in crop type and rotation has attracted increasing number of Eurasian cranes (Grus grus) to winter in the wetland (>40,000 in 2014). The birds are fed in an area of 100ha and roost during the night in the wetland for protection from predators. Feeding practices have yielded an eco-tourism bonanza with over 400,000 visitors annually. However, this practice may have negative impacts on the trophic state of the wetland. We performed trend analyses using monthly means of selected constituents collected from mid-1994 to 2014. The temporal distribution of TN and TP concentrations in the inlets did not change with time. The concentrations of TN and TP in the outlet increased significantly during the earlier monitoring period. Kendall–Theil regression showed that TP concentrations in the outlet increased significantly from a monthly mean of 180μgL−1 in 2010 to a monthly mean of 260μgL−1 in 2014. Similarly, the results of chlorophyll a concentrations in the outlet showed a sharp upturn in the latter part of the series from a mean of 66mgL−1 in 2010 to a mean of 122mgL−1 in 2014. The concurrent increase of TP and chlorophyll a, the two most important parameters affecting a waterbody trophic index coincided with the observed increase in the number of roosting cranes in the wetland. Hence, we assume that the continued increases in TP concentrations could transform the wetland from a mild eutrophic to a permanent hypereutrophic state. Reducing the number of roosting cranes may prevent this from happening.