Water column nutrients control growth and C:N ratios of symbiont‐bearing benthic foraminifera on the Great Barrier Reef, Australia

Abstract

In situ growth of two abundant diatom-bearing benthic foraminiferal species (Amphistegina radiata, Heterostegina depressa) was investigated to elucidate whether differences in abundance between inshore and offshore reefs are influenced by water quality. Five experiments were conducted over 2 yr, comparing growth rates of foraminifera at inshore coral reefs, which are exposed to terrestrial runoff, with those at reefs further offshore. Concentrations of most dissolved and particulate water quality variables were significantly higher inshore and during periods of high runoff (summer wet season). Foraminiferal growth was generally significantly lower on inshore reefs than offshore reefs, and growth of both species was reduced during the wet season. Depth transplantations confirmed that light was not an important factor in growth regulation. In contrast, multiple regression analyses of the effects of water quality variables on foraminiferal growth explained 69% of the variance in growth for A. radiata and 78% for H. depressa. Increased concentrations of dissolved nitrogen were associated with reduced growth. Intracellular carbon-to-nitrogen ratios in the foraminifera also reflected patterns in water quality, with generally lower values in foraminifera from inshore or during periods of high runoff, driven by higher intracellular N contents during these periods. It is suggested that increased nutrient availability releases foraminiferal symbionts from nutrient limitation. This may lead to reduced translocation of organic carbon to the host and resulting reduced host growth.

Temperature‐induced stress leads to bleaching in larger benthic foraminifera hosting endosymbiotic diatoms

“…To represent the environmental range under which these foraminifera are found naturally in the study area Uthicke and Altenrath 2010), samples were collected on two field trips on 06 August 2009-10 August 2009 and 16 January 2010-17 January 2010 in shallow water at 6-8 m and at 10-12 m LAT from two reefs close to the coastline (distance: 10-15 km, Daydream Island S 20u15.359, E 148u48.739, Double Cone Island) and two outer shore islands (distance: 30-37 km, Deloraine, Edward, and Border Islands). Specimens from the coastal islands are subjected to distinctly lower light levels and higher concentrations of dissolved inorganic nitrogen and suspended sediments compared to specimens from the outer shore islands (Cooper et al 2007;Uthicke and Altenrath 2010). Foraminifera were sampled and identified as described above.…”

“…Chl a analysis-To quantify changes in the symbiont population (as a proxy for biomass) the analysis for the total Chl a content of the holobiont was developed by modifying existing protocols for freshwater phytoplankton and marine sediment biofilms for LBF (Sartory and Grobbelaar 1984;Uthicke 2006). Foraminiferan samples were kept in low light conditions and on ice during analysis.…”

“…The algal symbiosis in corals and LBF share a common characteristic: both are well adapted to nutrientpoor waters of tropical coral reefs (Hallock 1981). This characteristic makes LBF good indicators of water quality because symbiotic species dominate the foraminiferal assemblages in nutrient-poor water and are less abundant under elevated nutrient levels (Foraminifera in Reef Assessment and Monitoring Index; Hallock et al 2003;Uthicke and Nobes 2008;Uthicke et al 2010). In situ experiments found that growth rates of A. radiata and H. depressa are negatively affected by elevated nutrient levels occurring in the GBR (Uthicke and Altenrath 2010).…”

“…This characteristic makes LBF good indicators of water quality because symbiotic species dominate the foraminiferal assemblages in nutrient-poor water and are less abundant under elevated nutrient levels (Foraminifera in Reef Assessment and Monitoring Index; Hallock et al 2003;Uthicke and Nobes 2008;Uthicke et al 2010). In situ experiments found that growth rates of A. radiata and H. depressa are negatively affected by elevated nutrient levels occurring in the GBR (Uthicke and Altenrath 2010). Therefore, hypotheses about the effects of nutrient stress on corals could apply to LBF as well: nutrient stress releases the symbionts from their nutrient limitation, subsequently leading to extensive population growth and reduced carbon translocation to the host (Dubinsky and Jokiel 1994;Dubinsky and Berman-Frank 2001).…”

Combined effects of warming and ocean acidification on coral reef Foraminifera Marginopora vertebralis and Heterostegina depressa

“…Growth of H. depressa and M. vertebralis was expressed as the increase in cross-sectional surface area per day over the experimental period. Procedures of high-resolution photography before and at the end of the experiment were the same as previously published (Uthicke and Altenrath 2010;Vogel and Uthicke 2012). Surface area (mm 2 ) of individual Foraminifera was measured and analysed as described in .…”