In order for euxinic conditions to persist, a positive feedback loop must perpetuate organic matter export to bottom waters and reduction of sulfate under anoxic conditions. Organic matter export is driven by high levels of primary production in the photic zone, supported by a continual supply of nutrients to the oxic surface waters. A natural source of nutrients, such as phosphate (), comes from weathering of rocks and subsequent transport of these dissolved nutrients via rivers. In a nutrient trap, increased input of phosphate from rivers, high rates of recycling of phosphate from sediments, and slow vertical mixing in the water column allow for euxinic conditions to persist.