Inhibited growth in the photosymbiont-bearing foraminifer Marginopora vertebralis from the nearshore Great Barrier Reef, Australia

Growth rates of benthic photosymbiont-bearing foraminifera are strongly influenced by nutrient concentrations and temperature, yet little is known about the effect of terrestrially derived nutrients on growth rates of foraminifera in the field. This study investigates the growth response of Marginopora vertebralis to enhanced terrestrially derived nitrogen and phosphate under both field and laboratory conditions. The field studies were conducted around the Whitsunday Islands in the Great Barrier Reef (GBR), Australia, once during a peak flooding period and once during the dry season. During the flood season, dissolved inorganic nitrogen and particulate phosphate increased 2- to 5-fold from terrestrially derived runoff from the Proserpine River catchment compared to the dry season. Increased exposure to nutrients derived from terrestrial runoff negatively affected the growth rate of M. vertebralis in the field and in a comparative laboratory experiment. The field study showed a clear effect of site proximity to the river mouth and the intensity of the river discharge. Growth rates were higher in the reduced terrestrial runoff conditions at the offshore sites. When exposed to naturally enriched dissolved inorganic nitrogen and phosphate in the laboratory experiment, growth rate of M. vertebralis was reduced 51 and 37 %, respectively, compared with ambient controls. Eutrophication likely causes destabilisation in the coupling of photosynthesis and growth within the photosymbiont by restricting photosynthate translocation to the host. In addition to nutrient effects, we also tested the effect of summer (28 degrees C) and winter (22 degrees C) temperatures on foraminiferal growth in the laboratory. Growth rates were significantly lower in the warmer treatment than in the cooler treatment. Therefore, both increased temperatures and higher nutrient concentrations appear to negatively affect growth in photosymbiont-bearing benthic foraminifera. Overall, management of local stressors (in this case nutrient runoff) might ameliorate the impact of climate change (increased temperature) on calcifying photosymbiont-bearing organisms.