Patterns of magnesium content in Arctic bryozoan skeletons along a depth gradient

Abstract

A growing body of evidence suggests that ocean acidification acting synergistically with ocean warming alters carbonate biomineralization in a variety of marine biota. Magnesium often substitutes for Ca in the calcite skeletons of marine invertebrates, increasing their solubility. The spatioenvironmental distribution of Mg in marine invertebrates has seldom been studied, despite its importance for assessing vulnerabilities to ocean acidification. Because pH decreases with water depth, it is predicted that levels of Mg in calcite skeletons should also decrease to counteract dissolution. Such a pattern has been suggested by evidence from echinoderms. Data on magnesium content and depth in Arctic bryozoans (52 species, 103 individuals, 150 samples) are here used to test this prediction, aided by comparison with six conceptual models explaining all possible scenarios. Analyses were based on a uniform dataset spanning more than 200 m of coastal water depth. No significant relationship was found between depth and Mg content; indeed, the highest Mg content among the analyzed taxa (8.7 % mol MgCO 3 ) was recorded from the deepest settings ([200 m). Our findings contrast with previously published results from echinoderms in which Mg was found to decrease with depth. The bryozoan results suggest that ocean acidification may have less impact on the studied bryozoans than is generally assumed. In the broad context, our study exemplifies quantitative testing of spatial patterns of skeletal geochemistry for predicting the biological effects of environmental change in the oceans.

Skeletal carbonate mineralogy of Scottish bryozoans

“…The main environmental control usually discussed is temperature [ 14 , 15 , 20 , 29 , 80 , 90 ], with higher temperatures reported to drive higher Mg-calcite and aragonite deposition. Other environmental factors which have also been shown to influence mineralogy include salinity [ 47 ], depth [ 40 , 61 ], aragonite compensation depth (ACD) [ 91 ], Mg/Ca ratio in seawater [ 80 , 81 , 88 , 90 , 92 ] and general seawater chemistry [ 29 , 87 ]. Many of these environmental factors, such as temperature, salinity, Mg/Ca ratio and ACD vary with latitude and have resulted in reported correlations between latitude and skeleton mineralogy [ 20 , 43 , 93 ].…”

Biomineralization in bryozoans: present, past and future

“…The potential vulnerability of bryozoans to OA has been highlighted in several studies that have hypothesized taxon responses in future oceans on the basis of knowledge of the mineralogy and geochemistry of recent and fossil species (Smith, , ; Taylor et al ., ; Borszcz et al ., ). However, rigorous experimental studies have been very limited.…”

“…14 mol% MgCO 3 : Gordon et al ., ). Borszcz, Kuklinski & Taylor () were unable to find the predicted decrease in the more soluble Mg‐rich skeletons with depth in bryozoan assemblages from the Arctic in response to the lower pH levels and greater vulnerability to dissolution.…”

Geochemical Proxies for Estimating Faunal Exposure to Ocean Acidification

“…Bryozoans (Myriapora truncata) transplanted to sites of CO 2 venting exhibited reduced Mg incorporation at acidic sites (mean pH 7.43) relative to controls (mean pH 8.07) (Lombardi et al, 2010). However, recent analysis of 53 species of Antarctic bryozoans showed no relationship of Mg/Ca to water depth, despite a 0.4 unit decline in pH with depth (Borszcz et al, 2013). With decreasing Ω arag , the Mg/Ca ratio in the tubes of serpulid polychaetes increases, whereas it declines in red coralline algae; notably, Mg/Ca in these taxa exhibits a much larger response to seawater temperature and salinity than to Ω arag (Ries, 2004(Ries, , 2006(Ries, , 2011.…”