External sources of nutrients

Abstract

The primary source of nitrogen to Lake Kinneret drainage basin is dissolved inorganic N originating in southern Europe and being carried over the Mediterranean Sea by the rain clouds. Evidence is presented that this nitrogen is cycled within the mountainous aquifers eventually outflowing to the Jordan River and later to Lake Kinneret as nitrate. Within the watershed, there are additional sources of nitrogen that add almost half of the total annual riverine load. These include “old” N previously buried in peat within the historic Hula Lake and marshes as well as N in fertilizers and N resulting from cattle excretion. The drying out of the Hula Valley water bodies some 60 years ago resulted in the oxic degradation of this peat and the conversion of organic N to NH4+ which is later nitrified and the NO3 product is transported in the watershed waterways. At some stage, particularly during the 1970s, peat dust was spread over all of the drainage basin, including Lake Kinneret, through aeolian transport. This is indicated by high levels of organic C and N in the northern tributaries and in the Jordan River itself as well as in Lake Kinneret. As a result of the additional respiratory processes induced by the large amounts of this allochthanous organic matter, those waterways and Lake Kinneret surface water showed lower pH and dissolved oxygen (DO) levels than in the following decades. The two main external sources of phosphorus to Lake Kinneret are riverine and dust. In accordance with water discharge, the riverine source predominates in winter, while the dust source is more important in the dry season that follows. Isotopic (oxygen) evidence suggests that most of the P in dust originates from P fertilization of cultivated soils. Notice that in this relatively dry climate, aeolian transport of nutrients plays an important role, particularly in summer.