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  • Sedimentary Geology 346 (2016) 1724

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    Sedimentary Geology

    j ourna l homepage: www.e lsev ie r .com/ locate /sedgeo

    Sediment generation by Halimeda on atoll interior coral reefs of thesouthern Maldives: A census-based approach for estimatingcarbonate production by calcareous green algae

    Chris T. Perry , Kyle M. Morgan, Michael A. SalterGeography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK

    Corresponding author.E-mail address: [email protected] (C.T. Perry). 2016 Elsevier B.V. All rights reserved.

    a b s t r a c t

    a r t i c l e i n f o

    Article history:Received 14 October 2016Accepted 23 October 2016Available online 27 October 2016

    Editor: B. Jones

    Methods for quantifying rates and size fractions of carbonate sand production on reefs remain limited, despite theurgent need for such data to support assessments of reef island and tropical beach-dominated shoreline resil-ience. Here we present a census-based approach that supports estimates of sediment generation by the calcare-ous green alga Halimeda spp., which is an often conspicuous reef and lagoon substrate coloniser. Based on datafrom Kandahalagala (South Huvadhoo atoll, southern Maldives), we present carbonate sediment productiondata for the two dominant Halimeda spp. (Halimeda macrophysa and Halimeda micronesica) that occur on thereef flat and reef slope habitats. Whilst total mean production rates by Halimeda spp. are similar in both habitats(reef flat average, 67.49 g CaCO3m2 yr1; reef slope, 70.89 g), individual species contributions differ markedly.H. micronesica dominates on the reef flat (annual mean 41.91 g CaCO3 m2 yr1, compared to 25.08 g byH. macrophysa), whilst production is dominated by H. macrophysa on the reef slope (H. macrophysa 40.49 g, H.micronesica 29.01 g CaCO3 m2 yr1,). In terms of sediment generation we show that these species also contrib-ute very differently to the sediment reservoir. Whilst the sedimentary breakdown products from H. micronesicaare somewhat bimodal (~17% is in the medium to very coarse sand fraction, and ~76% in the silt and clayfraction), almost all (N90%) of the segments produced byH.macrophysa rapidly degrade to silt and clay sized sed-iment. Based on our census data this suggests that Halimeda spp. will contribute only between 7 and9 g m2 yr1 of sand grade sediment on the reef flat and shallow slope habitats, but ~5560 g m2 yr1 ofmud grade sediment. Scaled to the total area of combined reef habitat around Kandahalagala (~130,583 m2)this equates to Halimeda spp. producing ~2192 kg of sand-grade sediment, but ~15,181 kg of mud-grade sedi-ment per year. However, sediment compositional data suggests that Halimeda sp. are actually a very minor con-stituent of reef and islands sediments, especially in the fine sediment fractions. This 1) suggests that much of thepredominantlyfiner-grained sediment generated byHalimeda has little actual relevance to the later stages of reefisland development, and 2) highlights the potential for marked discrepancies in terms of sediment generatingreef species abundance and resultant sediment generation rates, especially in terms of the types and sizes of sed-iment that are appropriate to supply adjacent beaches and islands.

    2016 Elsevier B.V. All rights reserved.

    Keywords:Carbonate sandCoral reefReef islandHalimedaCarbonate productionCarbonate mud

    1. Introduction

    There is major on-going debate about the resilience of low-lyingcoral reef islands and of carbonate-dominated tropical beaches to cli-mate and sea-level change (e.g., Wetzel et al., 2012; McLean andKench, 2015), with an acknowledgement that carbonate sediment sup-ply (as dictated by the ecology of the adjacent corals reefs) has a key roleto play in this respect (Perry et al., 2011). However, our ability to relatereef species abundance data tomeasures of sediment generation, and toquantify the sediment grain size fractions that result (and the durabilityof these constituents), remains limited. One visually conspicuouscoloniser of both coral reef framework and lagoonal sands is the

    calcifying green algaeHalimeda spp. which, when present in high densi-ties, has the potential to produce carbonate at very high ratese.g., reportedly in excess of 2 kg CaCO3 m2 yr1 (Drew, 1983; Freileand Hillis, 1997). The body (thallus) of Halimeda spp. plants comprisesa number of calcified segments which progressively calcify throughtime. This process results initially from the precipitation of needle-shaped aragonite inside the segments, many of which can rapidly alterto anhedral equant aragonite nanograins in living specimens, althoughthis may depend on species and environmental conditions (Macintyreand Reid, 1995). Post-mortem, the plant segments are released intothe surrounding sediment reservoir, initially contributing individualplates to the coarse sand to gravel grade size fractions (i.e., the 14 mm fraction), but with a proportion then disaggregating into mud-grade (b63 m) carbonate sediment as needles and nanograins are lib-erated (Folk and Robles, 1964; Neumann and Land, 1975; Ford and[email protected] logo

  • 18 C.T. Perry et al. / Sedimentary Geology 346 (2016) 1724

    Kench, 2012). As a result, Halimeda spp. can contribute to a wide rangeof sediment size fractions across reef sedimentary systems (Hewins andPerry, 2006), and some of this sediment may also form extensivebioherms (Rees et al., 2007), or be appropriate to contribute to the de-velopment of adjacent reef-associated landforms (e.g., beaches, reefislands; see Kench et al., 2005; Morgan and Kench, 2016).

    However, at least 44 species of Halimeda have been described acrosstropical reef-building regions (Guiry, 2016). These exhibit a diverserange of growth morphologies, which differ in terms of segment sizes(Hillis-Colinvaux, 1980), and occupy a wide variety of habitats thatrange across sediment-dominated lagoons to deep fore-reef slopesand walls. For example, some species form small, compact bushes thatgrow semi-cryptically on hard substrates producing either small (~510 mm diameter) segments e.g., Halimeda micronesica, or larger (to~20 mm) segments e.g. Halimeda tuna. Other species form tall (up to1015 cm), erect plants that have larger segments (up to 10 mm diam-eter) and holdfasts enabling a sand-dwellingmode of life (e.g. Halimedaincrassata, Halimeda monile), whilst others form elongate, hangingplants on the surfaces of walls and caves, some with very smallsegments (to ~5 mm diameter) (e.g. Halimeda goreauii), others withsegments exceeding 30mm in diameter (e.g.Halimeda gigas). Althoughnot well quantified there also appears to be considerable inter-speciesvariability in terms of plant growth and turnover rates (Multer andClavijo, 1989) and in terms of the degree of segment calcification(Hillis-Colinvaux, 1980; Freile and Hillis, 1997).

    Based on the above observations it is thus reasonable to hypothesisethat both overall carbonate production rates, and the types (and sizefractions) of sedimentary carbonate produced by different Halimedaspp. may vary significantly, which has evident implications for estimat-ing reef carbonate budgets and understanding carbonate sediment sup-ply. However, such inter-species variability remains poorly constrained.For example, data on Halimeda production rates derive, largely for fieldlogistical reasons, overwhelmingly from studies of the common sand-dwelling species H. incrassata (e.g., Wefer, 1980; Multer, 1988; Payri,1988; Freile, 2004), with data from other species that occupy very dif-ferent habitats being either sparse or entirely absent. Even more poorlyconstrained is our knowledge of the sedimentary breakdown productsfrom Halimeda spp. which derive, to the best of our knowledge, from asingle study on this same species (H. incrassata) (Neumann and Land,1975), with the strongly bimodal grain-size data they presented beingassumed representative of the genera as a whole. However, given therelevance of Halimeda spp. to overall carbonate sediment budgets(e.g., Drew, 1983), and that ongoing ecological and environmentalchanges may increase Halimeda plant abundances (e.g., Shulman andRobertson, 1996; Ferrari et al., 2012) and possibly alter the relativeabundances of Halimeda spp., this is a major knowledge gap. Specifical-ly, it highlights the need not only for an improved understanding of thetypes and rates of carbonate sediment being produced by differentHalimeda species, but also the merits of developing non-destructivemethodologies to enable production rates to be estimated from censusdata across different reefal habitats and by different species ofHalimeda.Here we present: 1) a census-based approach to measuring rates ofcarbonate sediment generation by Halimeda spp.; and 2) provide dataon the sediment grain-size proportions generated by differentHalimedaspp. These data were collected from the reefs around KandahalagalaIsland (South Huvadhoo atoll, southern Maldives) and have beenintegrated with field census data to support estimates of the rates andtypes of sediment generated by the two dominant lithophitic (hard-substrate dwelling) species that occur on the reef flat and shallowfore-reef habitats, Halimeda macrophysa (Askenasy) and H. micronesica(Yamada).

    2. Study area and methodology

    Field data and samples of H. macrophysa and H. micronesica (deter-mined from initial field investigations to be the two major shallow

    water species of Halimeda at this site) were collected from the r