Ammonia nitrogen dynamics in coastal rockpools affected by gull guano

Abstract

Summertime ammonia (including ammonia and ammonium) concentrations in infrequently flushed coastal rockpools located near gull rookeries (Appledore Island, Maine, USA) were found to change by several orders of magnitude over temporal scales of hours to days. Concentration increases were brought about by transport of rain-washed gull guano into the pools (rain-event changes: from near zero to 1600 μM) and nighttime ammonia release by pool biota (daily changes: from near zero to 60 μM). Fresh gull guano was found to release ammonia and phosphate rapidly in water, resulting in a molar ratio (N:P) of about 3.0 to 3.2, values similar to those observed in pools after a rainfall event. The rapid daytime losses occurred due to both assumed metabolic nutrient uptake, and ammonia gas volatilization during midday periods of high pH associated with photosynthetic oxygen production. Diel pH ranged from nighttime lows of 6.6–7.0 to daytime highs of 10.2–10.5. Daytime ammonia loss rates ranged from 130 to 250 μmol · l −1 · h −1 which is equivalent to a volatilization flux of 1–2 μmol · cm −2 · h −1 depending on the area and depth of the pool. Wind-aided gaseous flux across the pool surface is estimated to comprise ≈ 80% of observed ammonia loss, the remainder being lost through either direct metabolic uptake or infrequent flushing with seawater. Like most marine systems, these pools may become nitrogen limited despite the abundance of nitrogen at times. Because of the rapid ammonia volatilization rates, it appears that some of the nitrogen deposited in gull rookeries may be widely dispersed downwind of the rookery site. Volatilized ammonia may be further scrubbed from the air by precipitation, thus contributing to the eutrophication of adjacent and remote marine coastal and terrestrial ecosystems.