Reef Community Composition Research Articles

Bacterial Associates of Two Caribbean Coral Species Reveal Species-Specific Distribution and Geographic Variability

Scleractinian corals harbor microorganisms that form dynamic associations with the coral host and exhibit substantial genetic and ecological diversity. Microbial associates may provide defense against pathogens and serve as bioindicators of changing environmental conditions. Here we describe the bacterial assemblages associated with two of the most common and phylogenetically divergent reef-building corals in the Caribbean, Montastraea faveolata and Porites astreoides. Contrasting life history strategies and disease susceptibilities indicate potential differences in their microbiota and immune function that may in part drive changes in the composition of coral reef communities. The ribotype structure and diversity of coral-associated bacteria within the surface mucosal layer (SML) of healthy corals were assessed using denaturing gradient gel electrophoresis (DGGE) fingerprinting and 454 bar-coded pyrosequencing. Corals were sampled at disparate Caribbean locations representing various levels of anthropogenic impact. We demonstrate here that M. faveolata and P. astreoides harbor distinct, host-specific bacteria but that specificity varies by species and site. P. astreoides generally hosts a bacterial assemblage of low diversity that is largely dominated by one bacterial genus, Endozoicomonas, within the order Oceanospirillales. The bacterial assemblages associated with M. faveolata are significantly more diverse and exhibit higher specificity at the family level than P. astreoides assemblages. Both corals have more bacterial diversity and higher abundances of disease-related bacteria at sites closer to the mainland than at those furthest away. The most diverse bacterial taxa and highest relative abundance of disease-associated bacteria were seen for corals near St. Thomas, U.S. Virgin Islands (USVI) (2.5 km from shore), and the least diverse taxa and lowest relative abundance were seen for corals near our most pristine site in Belize (20 km from shore). We conclude that the two coral species studied harbor distinct bacterial assemblages within the SML, but the degree to which each species maintains specific microbial associations varies both within each site and across large spatial scales. The taxonomic scale (i.e., phylum versus genus) at which scientists examine coral-microbe associations, in addition to host-elicited factors and environmental fluctuations, must be considered carefully in future studies of the coral holobiont.

Implications of reef ecosystem change for the stability and maintenance of coral reef islands

AbstractCoral reef islands are among the most vulnerable environments on Earth to climate change because they are low lying and largely constructed from unconsolidated sediments that can be readily reworked by waves and currents. These sediments derive entirely from surrounding coral reef and reef flat environments and are thus highly sensitive to ecological transitions that may modify reef community composition and productivity. How such modifications – driven by anthropogenic disturbances and on‐going and projected climatic and environmental change – will impact reef island sediment supply and geomorphic stability remains a critical but poorly resolved question. Here, we review the unique ecological–geomorphological linkages that underpin this question and, using different scenarios of environmental change for which reef sediment production responses can be projected, explore the likely resilience of different island types. In general, sand‐dominated islands are likely to be less resilient than those dominated by rubble grade material. However, because different islands typically have different dominant sediment constituents (usually either coral, benthic foraminifera or Halimeda) and because these respond differently to individual ecological disturbances, island resilience is likely to be highly variable. Islands composed of coral sands are likely to undergo major morphological change under most near‐future ecological change scenarios, while those dominated by Halimeda may be more resilient. Islands composed predominantly of benthic foraminifera (a common state through the Pacific region) are likely to exhibit varying degrees of resilience depending upon the precise combination of ecological disturbances faced. The study demonstrates the critical need for further research bridging the ecological–geomorphological divide to understand: (1) sediment production responses to different ecological and environmental change scenarios; and (2) dependant landform vulnerability.

Characterizing sensitivity and uncertainty in a multiscale model of a complex coral reef system

Abstract Sensitivity and uncertainty are intrinsic properties of ecological models, and their characterization is an important step in the modelling process. We use a spatially explicit multi-scale model of a coral reef system to explore four aspects of model sensitivity and uncertainty: (i) sensitivity to initial conditions; (ii) sensitivity to parameter values; (iii) sensitivity to spatio-temporal resolution; and (iv) the effects of uncertainty about spatio-temporal variability of ecological processes on the shape of distributions of model predictions. We use reef community composition, visualized in multivariate space, as a response variable. This approach provides an easily interpretable representation of changes in reef community composition under different parameter conditions and spatio-temporal resolutions. It is also a useful means to visualize distributions of model outcomes under differing assumptions about the nature of variability in ecological processes in the real world. Our results indicate that reef state and recovery trajectories are particularly sensitive to parameters determining coral growth and mortality rates. Variability in model outcomes depends on assumptions about the way parameters vary in space and time, and is greater at local scales than at subregional and regional scales. We highlight the fact that predictions based on the kind of model presented here are likely to be more robust for subregions and regions than for particular reef localities.

Assessing the effects of disease and bleaching on Florida Keys corals by fitting population models to data

Coral diseases have increased in frequency over the past few decades and have important influences on the structure and composition of coral reef communities. However, there is limited information on the etiologies of many coral diseases, and pathways through which coral diseases are acquired and transmitted are still in question. Furthermore, it is difficult to assess the impacts of disease on coral populations because outbreaks often co-occur with temperature-induced bleaching and anthropogenic stressors. We developed spatially explicit population models of coral disease and bleaching dynamics to quantify the impact of six common diseases on Florida Keys corals, including aspergillosis, dark spots, white band, white plague, white patch, and Caribbean yellow band. Models were fit to an 8-year data set of coral abundance, disease prevalence, and bleaching prevalence. Model selection was used to assess alternative pathways for disease transmission, and the influence of environmental stressors, including sea temperature and human population density, on disease prevalence and coral mortality. Classic disease transmission from contagious to susceptible colonies provided the best-fit model only for aspergillosis. For other diseases, external disease forcing, such as through a vector or directly from pathogens in the environment, provided the best fit to observed data. Estimates of disease reproductive ratio values ( R 0) were less than one for each disease, indicating coral colonies were below densities required for diseases to become established through contagious spread alone. Incidences of white band and white patch disease were associated with greater susceptibility or slower recovery of bleached colonies, and no disease outbreaks were associated with periods of elevated sea temperatures alone. Projections of best-fit models indicated that, atleast during the period of this study, disease and bleaching did not have substantial impacts on populations and impaired rates of population growth appeared to be attributable to other stressors. By applying epidemiological models to field data, our study gives qualitative insights into the dynamics of coral diseases, relative stressor impacts, and directions for future research.

Reef-Building Corals and Reefs of Vietnam: 2. The Gulf of Tonkin

This paper deals with the history and results of the studies of reefs and coral communities of the Gulf of Tonkin based on published and unpublished materials, including the author's. The state of the art in the study of reef-building scleractinian corals and reefs of this region is reported. The peculiar nature of the reefs studied is caused by the monsoon climate in the region and river runoff waters cooled to 16–18°C, silted to 100 g/m2 per day, and freshened to 28‰ in the wintertime, i.e., conditions far from optimum for reef formation. The silting and eutrophication of the gulf waters resulted in a change in the composition and structure of the coral reef communities via the reduction or elimination of certain coral species. Instead of acroporids, typical for the majority of other reefs, reef communities of the Gulf of Tonkin are dominated by poritids and faviids, which form the framework of the reefs. These peculiarities make the reefs of the Gulf of Tonkin really unique.

Applied Paleoecology and the Crisis on Caribbean Coral Reefs

In a book review published in 1982, the late Peter Williamson characterized paleoecology as “a poor-man's applied ecology performed on inadequate data.” Whether or not this incendiary remark was justified at the time, there is no disputing that in the years since 1982 paleoecology has provided much insight into the nature of biotic interactions and community structure. As a recent and obvious example, the debate over coordinated stasis has forced us to consider macroecological dynamics at spatial and temporal scales greater than those of the community. Disagreement over these ideas has led to research on how and why fossil assemblages change through time. Paleoecology is clearly thriving as an intellectual pursuit. The first part of Williamson's remark is the least controversial. Most of us will indeed remain poor in terms of funding for basic research in paleoecology. If taken seriously, however, those stinging words pose an unintentional and ironic challenge: to use paleoecology to solve practical problems. Paleoecologists must direct their efforts to the pressing ecological challenges before humanity, and they must practice applied paleoecology with adequate—that is to say, statistically robust—data. One problem that is clearly important from a practical standpoint is the radically altered composition of reef communities in the Caribbean over the past 25 years. The regional-scale transition of Caribbean reef communities since the late 1970s from dominance by scleractinian corals to dominance by noncoralline (fleshy and filamentous) macroalgae has provoked strenuous debate about its underlying ecological processes. Is the current situation natural or the product of anthropogenic stresses and disturbances? Knowing whether similar changes occurred in the past is crucial to addressing this question. By now the wretched state of Caribbean coral reefs should be depressingly familiar. Reef habitats that once supported spectacular coral growth are now covered with brown algae. On the fore reef off …

Effects of herbivory, nutrient levels, and introduced algae on the distribution and abundance of the invasive macroalga Dictyosphaeria cavernosa in Kaneohe Bay, Hawaii

Since the 1960s, and possibly earlier, the macroalga Dictyosphaeria cavernosa has overgrown and displaced corals on reef slopes and outer reef flats in Kaneohe Bay, Oahu. This shift in reef community composition is generally attributed to nutrient enrichment resulting from sewage discharge. Following the diversion of most of the sewage effluent in 1977–1978, it was expected that D. cavernosa growth would become nutrient-limited and its abundance would consequently decline, but the alga remains abundant in much of the bay. One explanation for its persistence is that nutrients are once again high enough to support the alga's growth. An alternative explanation is that there has been a reduction in grazing intensity in the bay. In this study we resurveyed the distribution and abundance of D. cavernosa at 120 reef slope sites originally surveyed in 1969. We conducted additional surveys to estimate the biomass of herbivores and the areal coverage of D. cavernosa and other macroalgae on reef slopes and flats. Field experiments were used to determine spatial and temporal patterns of grazing intensity on and growth rates of D. cavernosa and the introduced macroalga Acanthophora spicifera. Laboratory experiments were used to examine preferences among herbivores for some of the most abundant macroalgae on Kaneohe Bay reefs. Twenty years after sewage diversion, D. cavernosa cover on reef slopes has decreased substantially in southern Kaneohe Bay, the site of most of the historical sewage discharge. D. cavernosa cover has changed less in other regions, remaining high in the central bay and low in the north bay. D. cavernosa thalli protected by grazer exclusion cages sustained positive growth rates on reef slopes and flats throughout the bay. Reduced nutrient concentrations may have caused a reduction in D. cavernosa growth rates, and a consequent reduction in D. cavernosa abundance in the south bay shortly after sewage diversion. Measurements of grazing intensity and surveys of herbivorous fish abundance suggest that the continued abundance of D. cavernosa is the result of a reduction in grazing intensity. Reduced grazing intensity on D. cavernosa may in turn be the result of a historical reduction in herbivore biomass or the establishment of several introduced macroalgae on reef flats. The introduced species are preferred by herbivorous fishes over D. cavernosa, as indicated by preference tests. The hypothesis that reduced grazing pressure on D. cavernosa is related to the establishment of introduced species is supported by the observation that D. cavernosa cover is highest on reef slopes where the cover of preferred introduced macroalgae on the adjacent outer reef flat is also high. Conversely, D. cavernosa cover is low or zero on reef slopes where the cover of introduced macroalgae on the adjacent reef flat is low or zero

Pleistocene reef environments, constituent grains, and coral community structure: Curaçao, Netherlands Antilles

We investigated the degree to which component grains vary with depositional environment in sediments from three reef habitats from the Pleistocene (125 ka) Hate Unit of the Lower Terrace, Curacao, Netherlands Antilles: windward reef crest, windward back reef, and leeward reef crest. The windward reef crest sediment is the most distinctive, dominated by fragments of encrusting and branching coralline red algae, coral fragments and the encrusting foraminiferan Carpenteria sp. Windward back reef and leeward reef crest sediments are more similar compositionally, only showing significant differences in relative abundance of coral fragments and Homotrema rubrum. Although lacking high taxonomic resolution and subject to modification by transport, relative abundance of constituent grain types offers a way of assessing ancient skeletal reef community composition, and one which is not limited to a single taxonomic group. The strong correlation between grain type and environment we found in the Pleistocene of Curacao suggests that constituent grain analysis may be an effective tool in delineating Pleistocene Caribbean reef environments. However, it will not be a sufficient indicator where communities vary significantly within reef environments or where evolutionary and/or biogeographical processes lead to different relationships between community composition and reef environment. Detailed interpretation of geological, biological, and physical characteristics of the Pleistocene reefs of Curacao reveals that the abundance of the single coral species, Acropora palmata, is not a good predictor of the ecological structure of the ancient reef coral communities. This coral was the predominant species in two of the three reef habitats (windward and leeward reef crest), but the taxonomic composition (based on species relative abundance data) of the reef coral communities was substantially different in these two environments. We conclude that qualitative estimates of coral distribution patterns (presence of a key coral species or the use of a distinctive coral skeletal architecture), when used as a component in a multi-component analysis of ancient reef environments, probably introduces minimal circular reasoning into quantitative paleoecological studies of reef coral community structure.

Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry

The primary mineralogy of oolites and early marine carbonate cements led Sandberg [Nature 305 (1983), 19–22] to divide the Phanerozoic Eon into three intervals of `aragonite seas' and two intervals of `calcite seas'. Hardie [Geology 24 (1996), 279–283] has shown that these oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular shifts in the Mg/Ca ratio of seawater driven by changes in spreading rates along mid-ocean ridges. The Hardie model also predicts that high-Mg calcite should precipitate along with aragonite, as it does in today's aragonite sea. We have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms (ones that have produced massive skeletons, large reefs, or voluminous bodies of sediment) that correspond to Sandberg's aragonite seas and calcite seas and that are predicted by the Hardie model. Particular groups of corals, sponges, and algae appear to have been dominant reef builders only when favored by an appropriate Mg/Ca ratio in seawater. In early and middle Paleozoic calcite seas (Calcite I), reefs were dominated by calcitic tabulate, heliolitid, and rugose corals and calcitic stromatoporoids. In contrast, during the period of late Paleozoic–early Mesozoic aragonite seas (Aragonite II), aragonitic groups of sponges, scleractinian corals, and phylloid algae, as well as high-Mg calcitic red algae, were principal reef builders. During Late Cretaceous time, at the acme of Calcite II, massive rudists displaced aragonitic hermatypic corals. In today's aragonite sea (Aragonite III) scleractinian corals are again dominant reef builders, along with high-Mg calcitic coralline algae. Major sediment-producing algae exhibit temporal patterns similar to those of reef builders. Calcitic receptaculitids flourished during Calcite I, whereas aragonitic dasycladaceans did not become dominant rock formers until Aragonite II. During Calcite II, calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the Late Cretaceous, after the Mg/Ca ratio of seawater had reached a very low value and calcium concentration, a very high value. As the Mg/Ca ratio of seawater rose and calcium concentration fell during the Cenozoic Era, individual coccoliths, on average, became less massive and encrusted cells less thickly. By Pliocene time, during Aragonite III, the prominent genus Discoaster secreted only narrow-rayed coccoliths that covered less than 25% of the cell surface. Also during Aragonite III, the aragonitic green alga Halimeda emerged as the dominant skeletal sediment producer in reef tracts. The influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification. Such groups include algae, sponges, corals, and bryozoans. Morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs. This linkage, in addition to the basic chemical demands of hypercalcification, has given the Mg/Ca ratio of seawater strong control over the success of individual reef-building taxa. More generally, this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history. We conclude that throughout Phanerozoic time a chain of causation has extended from mid-ocean ridge processes, via seawater chemistry, to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits.

A late Devonian (Frasnian) coral-bafflestone reef at Houshan in Guilin, South China

Givetian to early Carboniferous sediments of South China are characterized by carbonates. Middle and Late Devonian strata are best developed in the Guilin area. Reefs and organic shoals are recorded by various lithofacies types indicating the existence of an extended carbonate platform and a change of the composition of reef communities in time. Starting in the late Devonian, stromatoporoids and corals were replaced by algae that subsequently played an important role together with stromatoporoids, receptaculitids and fasciculate rugose corals in reef communities. In Houshan, 5 km west of Guilin, a coral-bafflestone reef occurs in the Frasnian strata, situated near an offshore algal-stromatoporoid reef. The coral reef was formed in a back-reef area adjacent to the inner platform margin. The coral-bafflestone reef is unique among the late Devonian reefs of South China with regard to the biotic composition. The reef is composed of fasciculate colonies ofSmithiphyllum guilinense n. sp. embedded within in packstones and wackestones. The height of colonies reaches 1 m. The community is low-diverse. The species ofSmithiphyllum occurring in the Frasnian reef complexes of Guilin exhibit a distinct facies control:Smithiphyllum guilinense occurs in or near to margin facies and formed bafflestone, constituting a coral reef whereasSmithiphyllum occidentaleSorauf, 1972 andSmithiphyllum sp.—characterized by small colonies with thin corallites—are restricted to the back-reef and marginal slope facies. The bush-like coral colonies baffled sediments. Algae and stromatoporoids (mainlyStachyodes) are other reef biota. Reef-dwelling organisms are dominated by brachiopods.

Clipperton Atoll (eastern Pacific): oceanography, geomorphology, reef-building coral ecology and biogeography

Coral reef geomorphology and community composition were investigated in the tropical northeastern Pacific during April 1994. Three areas were surveyed in the Revillagigedo Islands (Mexico), and an intensive study was conducted on Clipperton Atoll (1,300 km SW of Acapulco), including macro-scale surface circulation, sea surface temperature (SST) climatology, geomorphology, coral community structure, zonation, and biogeography. Satellite-tracked drifter buoys from 1979–1993 demonstrated complex patterns of surface circulation with dominantly easterly flow (North Equatorial Counter Current, NECC), but also westerly currents (South Equatorial Current, SEC) that could transport propagules to Clipperton from both central and eastern Pacific regions. The northernmost latitude reached by the NECC is not influenced by El Nino-Southern Oscillation (ENSO) events, but easterly flow velocity evidently is accelerated at such times. Maximum NECC flow rates indicate that the eastern Pacific barrier can be bridged in 60 to 120 days. SST anomalies at Clipperton occur during ENSO events and were greater at Clipperton in 1987 than during 1982–1983. Shallow (15–18 m)and deep (50–58 m) terraces are present around most of Clipperton, probably representing Modern and late Pleistocene sea level stands. Although Clipperton is a well developed atoll with high coral cover, the reef-building fauna is depauperate, consisting of only 7 species of scleractinian corals belonging to the generaPocillopora, Porites, Pavona andLeptoseris, and 1 species of hydrocoral in the genusMillepora. The identities of the one Pocilpopora species and one of the twoPorites species are still unknown. Two of the remaining scleractinians (Pavona minuta, Leptoseris scabra) and the hydrocoral (Millepora exaesa), all formerly known from central and western Pacific localities, represent new eastern Pacific records. Scleractinian corals predominate (10–100% cover) over insular shelf depths of 8 to 60m, and crustose coralline algae are dominant (5–40% cover) from 0.5 to 7m. Spur and groove features, constructed of alternating frameworks ofPocillopora andPorites, and veneered with crustose coralline algae, are generally well developed around most atoll exposures. Although crustose coralline algae predominate in the breaker zone (with up to 100% cover), a prominent algal ridge is absent with only a slight buildup (ca. 10 cm) to seaward. Frequent grazing by the pufferfishArothron meleagris results in the removal of large amounts of live tissue and skeleton fromPorites lobata. Acanthaster planci is present, but rare. The grazing of large diadematid sea urchins, (2 species each ofDiadema andEchinothrix) on dead corals cause extensive erosion in some areas. Large numbers of corals on the 15–18 m terrace had recently suffered partial (P. lobata, 60–70% maximum of all colonies sampled) or total (Pocillopora sp., 80% maximum) mortality. The lengths of regenerating knobs and the rates of linear skeletal growth inP. lobata, determined by sclerochronologic analysis, indicated a period of stress during 1987. Massive skeletal growth is significantly higher at intermediate (16–17 m) than shallow (6–8 m) depths with mean extension rates of 1.5 mm yr−1 inP. lobata and 1.4 mm yr−1 inP. minuta at intermediate depths. Skeletal growth inP. lobata was depressed during the 1987 El Nifio event at Clipperton. The branching coralPocillopora sp. demonstrated high and similar skeletal growth rates at both shallow (25.4 mm yr−1) and intermediate (26.5 mm yr−1) depths. The presence of widely distributed Indo-Pacific zooxanthellate corals at Clipperton and the Revillagigedo Islands indicates that these NE Pacific Islands probably serve as a stepping stone for dispersal into the far eastern Pacific region.

Pleistocene Perspectives on Coral Reef Community Structure

Composition and zonation of coral reef communities are unstable on the scale of human lifetimes, but stable on average over thousands to hundreds of thousands of years. Traditional small-scale ecological studies can miss community patterns which are obscured by the noise of short-term change. Tests for stability require monitoring of populations over many generations—millenia for long-lived corals and forest trees. The only recourse is the fossil record.

Patterns of sediment composition of Jamaican fringing reef facies

ABSTRACTRecent carbonate sediments from Jamaican north coast fringing reefs were collected along three parallel traverses in the vicinity of Discovery Bay. Each traverse extended from near shore across the back reef, reef crest, and fore reef to a depth of 75 m. Relative abundances of the biotic constituents vary between sites, reflecting general patterns of reef community composition. The sediment is dominated by highly comminuted coral fragments (27·1% to 63·1%), plates of the calcareous green alga Halimeda (0·4% to 38·7%), coralline algae (4·7% to 16·2%) and the encrusting foraminiferan Homotrema rubrum (0·7% to 9·5%), with lesser amounts of other taxonomic groups (non‐encrusting foraminifera 1·3–5·5%; molluscs 1·4–7·0%; echinoderms 0·9–5·0%). Coral fragments, coralline algae and particles of Homotrema rubrum dominate the sediments of the shallow portions of the fore reef (5–15 m), whereas plates of Halimeda are most abundant in sediments from the back reef and deeper portions of the fore reef ( 24 m). Q‐mode cluster analysis, using sediment constituent data, resulted in the delineation of four reef biofacies over the depth range of this study (1–75 m).

Some aspects of the oceanography of Little Port Walter Estuary, Baranof Island, Alaska : Powers C.F., 1963. U.S. Fish. Wild. Serv., Fish. Bull., 63 (1): 143–164

The primary mineralogy of oolites and early marine carbonate cements led Sandberg [Nature 305 (1983), 19–22] to divide the Phanerozoic Eon into three intervals of `aragonite seas' and two intervals of `calcite seas'. Hardie [Geology 24 (1996), 279–283] has shown that these oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular shifts in the Mg/Ca ratio of seawater driven by changes in spreading rates along mid-ocean ridges. The Hardie model also predicts that high-Mg calcite should precipitate along with aragonite, as it does in today's aragonite sea. We have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms (ones that have produced massive skeletons, large reefs, or voluminous bodies of sediment) that correspond to Sandberg's aragonite seas and calcite seas and that are predicted by the Hardie model. Particular groups of corals, sponges, and algae appear to have been dominant reef builders only when favored by an appropriate Mg/Ca ratio in seawater. In early and middle Paleozoic calcite seas (Calcite I), reefs were dominated by calcitic tabulate, heliolitid, and rugose corals and calcitic stromatoporoids. In contrast, during the period of late Paleozoic–early Mesozoic aragonite seas (Aragonite II), aragonitic groups of sponges, scleractinian corals, and phylloid algae, as well as high-Mg calcitic red algae, were principal reef builders. During Late Cretaceous time, at the acme of Calcite II, massive rudists displaced aragonitic hermatypic corals. In today's aragonite sea (Aragonite III) scleractinian corals are again dominant reef builders, along with high-Mg calcitic coralline algae. Major sediment-producing algae exhibit temporal patterns similar to those of reef builders. Calcitic receptaculitids flourished during Calcite I, whereas aragonitic dasycladaceans did not become dominant rock formers until Aragonite II. During Calcite II, calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the Late Cretaceous, after the Mg/Ca ratio of seawater had reached a very low value and calcium concentration, a very high value. As the Mg/Ca ratio of seawater rose and calcium concentration fell during the Cenozoic Era, individual coccoliths, on average, became less massive and encrusted cells less thickly. By Pliocene time, during Aragonite III, the prominent genus Discoaster secreted only narrow-rayed coccoliths that covered less than 25% of the cell surface. Also during Aragonite III, the aragonitic green alga Halimeda emerged as the dominant skeletal sediment producer in reef tracts. The influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification. Such groups include algae, sponges, corals, and bryozoans. Morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs. This linkage, in addition to the basic chemical demands of hypercalcification, has given the Mg/Ca ratio of seawater strong control over the success of individual reef-building taxa. More generally, this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history. We conclude that throughout Phanerozoic time a chain of causation has extended from mid-ocean ridge processes, via seawater chemistry, to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits.