Certain red and green algal species respond with rapid growth to excess nitrogen inputs in estuaries. These seaweeds (e.g., Ulva or "sea lettuce", Cladophora, and Gracillaria) can produce prodigious amounts of plant matter along the bottom of shallow embayments subjected to high nutrient loads. Some of this material is "drift algae" that moves through the embayment with the currents. Macroalgae contribute to the variability of oxygen concentrations and introduce instability into the functioning of the estuarine ecosystem. Although macroalgal photosynthesis creates oxygen during sunny days, at night or on cloudy days the algal oxygen demand can outstrip oxygen availability, driving oxygen levels to deleteriously low ("hypoxic") or zero ("anoxic") levels. Massive fish kills are possible under these conditions, even if the anoxic event lasts only a few hours.