Bay Off Research Articles

Underwater soundscape in Seaview Bay, Antarctica, and triple ascending trill of the leopard seal (Hydrurga leptonyx) underwater vocalizations.

The underwater soundscape was recorded in Seaview Bay off Inexpressible Island, Ross Sea region Marine protected area, for 3 days in December 2021. Leopard seal Hydrurga leptonyx vocalizations were a prominent sound source that led to variations in ambient sound pressure levels in a frequency range of approximately 150-4500 Hz. Among the 14 call types previously identified, except ultrasound vocalizations, six types of broadcast calls were classified, and their acoustic characteristics were analyzed. We focused on the acoustic characteristics of four low-frequency calls, clustered in a relatively narrow bandwidth, which have been relatively less studied. We identified a new call type of a triple ascending trill consisting of three trill parts, expanding upon the findings of previous studies. The audio data extracted from leopard seal vocalization videos, recorded by a monitoring camera on sea ice, enhanced the reliability of identifications of the underwater triple ascending trill. We present the unique results of underwater passive acoustic monitoring conducted at Seaview Bay, designated as Antarctic Specially Protected Area No 178. Our results could contribute to the development of detection and localization algorithms for leopard seal vocalizations and can be used as fundamental data for studies related to the vocalization and behavior of this species.

Coral Reveals a Transition in Sources of Seawater Vanadium From Submarine Groundwater Discharge to Upwelling in the Northern South China Sea

AbstractIn recent decades, vanadium (V) levels in the environment have dramatically increased and will continue to rise alarmingly in the future. Conventional methods for assessing oceanic V typically only involved the collection of seawater and sediments, which significantly limited our understanding of the dynamics of oceanic V and its relationship to environment and climate. Here, the interannual temporal variations in surface seawater V levels in the northern South China Sea, were reconstructed utilizing a 137‐year record of V/Ca ratios from Porites lutea coral from Xiaodonghai Bay off Sanya. We observed that anthropogenic activities had a limited impact on the V levels of surface seawater in the study area, whereas V variability exhibited significant disparities across different time periods. Precipitation‐related submarine groundwater discharge (SGD) recharge was the primary driver of V variation prior to 1990, with tropical cyclones (TCs) playing a detectable role in total precipitation during this period. Conversely, changes in surface seawater V levels after 1990 were predominantly linked to increasingly intense upwelling driven by summer winds. The recharge processes in unconfined groundwater systems within the study area might generate favorable oxic and alkaline conditions, which facilitated mineral dissolution and/or desorption reactions that elevated V concentrations along the groundwater flow path, which were recorded in coral skeletal V/Ca ratios. This study confirmed the seawater V was dominated by SGD and upwelling near Sanya, providing essential evidence for comprehending the geochemical behavior of V and the impact of ocean‐atmospheric climate interactions on long‐term variations in seawater V levels.

Fisheries and Biology of Otolithes ruber (Bloch & Schneider, 1801) Catch of Small-scale Fisherfolk in San Miguel Bay, Philippines

Otolithesruber (Bloch & Schneider, 1801) is one of the major fish catches in San Miguel Bay and its processing is a seasonal livelihood among coastal villages. This study documented and described the O. ruber catch of small-scale fisherfolks around the Bay through on-board survey and observation. Fishing trips from 10 September to 19 November 2018 demonstrated that fisherfolks used bottom-set gill nets and commonly explored the middle to the mouth of the Bay off Mercedes and Siruma to catch O. ruber and other species. The analysis of 434 composite fish samples bought from the fishing trips identified 222 female and 212 male fish individuals with TL of 19.14 + 0.11 cm and body weight of 70.76 + 1.46 g. The majority were already mature to spawning stages and gonads had no apparent lesions. The population’s reproductive capacity, based on the presence of numerous spermatocytes and oocytes, was presumed to be realized, but this may fail due to uncontrolled illegal fishing.

<i>Markevitchielinus anchoratus</i> Titar, 1975 (Copepoda: Chondracanthidae) parasitic on sea raven <i>Hemitripterus villosus</i> (Pallas, 1814) (Actinopterygii: Hemitripteridae), with the range extension of the copepod in the Northwestern Pacific Ocean

Markevitchielinus anchoratus Titar, 1975 is reported based on an adult female from the floor of the buccal cavity of a sea raven Hemitripterus villosus (Pallas, 1814) caught in Onagawa Bay, an inlet of the Northwestern Pacific Ocean, Miyagi Prefecture, northern Japan. The female specimen is briefly described. The female inserted its head with a pair of large, lateral processes and the anterior part of an elongate neck into the tissues of the buccal cavity floor. The present collection of M. anchoratus represents the third occurrence record of the species and extends its distributional range from Kunashiri and Shikotan islands, east of Hokkaido, to Onagawa Bay off Honshu, the largest main island of Japan. The species is specific to the sea raven and may be a subarctic species.

Assessing the potential use of environmental DNA for multifaceted genetic monitoring of cetaceans: Example of a wandering whale in a highly disturbed bay area

Environmental DNA (eDNA) sampling of water is a powerful method for comprehensive and noninvasive monitoring of aquatic animal species. However, there have been few reports of its application to cetacean species. On June 29, 2021, a whale (nicknamed Xiaobu) appeared in Dapeng Bay off Guangdong Province, China. We used eDNA technology to obtain information related to this whale (e.g., species identification and food resources) and to trace its possible origin. Fragments of four whale mitochondrial sequences (12S rDNA, 16S rDNA, cytochrome c oxidase subunit 1, and control region) were obtained from amplicons of eDNA collected in Dapeng Bay; sequence barcoding showed that this was an Eden’s whale (Balaenoptera edeni edeni Anderson 1879). Analysis of potential prey species (PPS) suggested that this whale might enter Dapeng Bay while tracking prey, mainly sardines (Sardinella lemuru, Sardinella gibbosa and Sardinella jussieui) and anchovies (Thryssa dussumieri, Thryssa vitrirostris and Thryssa kammalensis). Retrieval of eDNA metabarcoding data from samples collected in waters adjacent to Dapeng Bay (i.e. Lingding Bay and Daya Bay) revealed that Eden’s whale had appeared outside Lingding Bay up to 2 months prior to the appearance of this whale in Dapeng Bay (early April 2021). Overall, this study showed that eDNA is a highly effective noninvasive survey method for the accurate identification of target cetacean species and prey composition; it can be used to monitor megafauna that are under strict legal protection or to monitor megafauna with unknown conditions.

Inter-annual variability in growth and maturation of the swordtip squid Uroteuthis edulis in Yilan Bay off northeastern Taiwan

The swordtip squid Uroteuthis edulis, distributed in the temperate and tropical neritic waters in the Indo-Pacific region, is a crucial component of marine ecosystems and is a target species of fisheries in the Northwest Pacific. Although studies have elucidated the growth and maturation of the squid in the southern East China Sea (ECS) shelf, few have assessed the population biology of the squid in waters in the vicinity of the southern ECS. We collected samples of swordtip squid from a trawler in Yilan Bay (YLB) between January 2011 and December 2012 to examine the growth and maturation of the squid based on statolith microstructures and the inter-annual variability in certain parameters as well as the potential underlying mechanisms. The average dorsal mantle length (ML) of the squid was significantly higher in 2011 than in 2012 in both sexes. The ML-body weight relationship differed significantly between 2011 and 2012 in both sexes. Squid hatching peaked in summer (June and August)in 2011 and spring (March and April) in 2012. The average ML-at-age data were described using the exponential function and power function for female and male squid, respectively. Female squid matured to 139 mm at 133 days in 2011 and to 133 mm at 137 days in 2012. Male squid matured to 160 mm at 151 days in 2011 and to 148 mm at 133 days in 2012. However, the ML- and age-at-maturity of the squid in YLB were lower than those of the squid in the southern ECS shelf. These results indicate that YLB is one of the feeding grounds for squid that have hatched in the southern ECS shelf and have been transported southward intermittently to YLB. Further study of the population structure and potential population connectivity of the squid in the entire ECS shelf is warranted to obtain data for the development of effective conservation and management measures for the squid and its fisheries in the Northwest Pacific.

Effects of environmental factors on the detection of subsurface green turtles in aerial drone surveys

Abstract Context Aerial video surveys from unpiloted aerial systems (UAS) have become popular in wildlife research because of increased accessibility to remote areas, reduction of anthropogenic disruption to habitats and wildlife, low operating costs, and improved researcher safety. In shallow marine systems, they can provide opportunities to rapidly survey species that cannot easily be surveyed using boat- or land-based techniques. However, detectability of subsurface animals in marine habitats may be affected by environmental factors. Aims We investigated the effects of water depth, seagrass cover, surface glare, and observer numbers and expertise on the probability of detecting subsurface green turtles in UAS video surveys. Methods We deployed inanimate green turtle decoys at randomised intervals along 24 pre-determined transects across a depth gradient in a seagrass-dominated bay off Great Abaco, The Bahamas. We collected aerial videos of the transects by flying a DJI Phantom 3 Advanced quadcopter drone at an altitude of 10 m over each transect. Three independent observers watched each video and recorded decoy sightings to compare detection probabilities across observer experience levels. We used a generalised linear model to test for the effects of glare, water depth, wind speed, and seagrass cover on the detectability of turtle decoys. We also recorded glare conditions with aerial videos taken at 2-h intervals over a still body of water on cloudless days off North Miami, FL. Key results Individual observers performed similarly, but adding one additional observer increased detection by 11–12% and adding a third observer increased detections by up to 15%. Depth, seagrass cover, and glare significantly affected decoy detections. In both summer and fall, the optimal times and directions to minimise glare in aerial video surveys were 0800 hours, facing any direction other than north, and 1800 hours, facing any direction other than south. Conclusions The number of human observers and environmental variables, especially depth, seagrass cover, and glare, are important to explicitly consider when designing and analysing data from UAS surveys of subsurface animal abundances and distribution. Implications Our study draws attention to potential limitations of UAS-acquired data for subsurface observations if environmental conditions are not explicitly accounted for. Quantifying the effects of environmental factors, designing surveys to minimise variance in these factors, and having multiple observers are crucial for optimising UAS use in research and conservation of sea turtles and other marine fauna.

Investigating Spinner Dolphin (Stenella longirostris) Occurrence and Acoustic Activity in the Maui Nui Region

Hawaiian spinner dolphins (Stenella longirostris) rest during the day in the islands’ coastal waters where they are susceptible to human disturbance. Due to concerns over the negative impacts of human activity, the Pacific Islands Regional Office of the National Oceanic and Atmospheric Administration (NOAA) has proposed a 50-yard approach rule for spinner dolphins and is also considering time-area closures of certain important spinner dolphin resting areas. However, gaps still persist in the understanding of how spinner dolphin populations on different Hawaiian Islands use coastal waters, raising questions about the efficacy of the proposed rules. To better understand how spinner dolphins use the waters in the Maui Nui region (Maui, Moloka‘i, Lāna‘i, and the ‘Au‘au channel), a study was conducted using a combination of passive acoustic monitoring and vessel surveys to document spinner dolphin occurrence and movements. Bottom-moored acoustic recorders were deployed at eight locations in Maui Nui, and at one previously established resting bay off west O‘ahu for comparison. The amount of whistles, clicks, and burst pulses at each location was quantified and averaged by the hour of the day. Acoustic activity was greater at the O‘ahu site than at any of the Maui Nui sites, and was greatest between sunrise and noon. Acoustic activity and vessel surveys both reveal that spinner dolphins occur and exhibit resting behaviors in the ‘Au‘au channel between Maui and Lāna‘i, and also along west Maui and southeast Lāna‘i. Spinner dolphins resting in a channel between islands appears to be unique to Maui Nui and differs from resting patterns described along Hawai‘i Island and O‘ahu. Because spinner dolphins appear to use both the coastlines and the channel to rest in Maui Nui, the 50-yard approach rule appears to be a more suitable management option for the region than time-area closures.

Algae bloom phenomenon in Jakarta Bay as symptoms of severe eutrophication: Monitoring results of 2014-2016

Algae bloom is one of the symptoms of eutrophication process in an estuary, causing in some ecological and societal problems for the aquatic environments and human activities. Algae blooms observation was conducted in the dry season period between 2014 to 2016, in an eutrophic embayment of Jakarta Bay, Indonesia. Observations were made by conducting transect-boat survey and plankton sampling once the bloom was observed. Samples of chlorophyll-a and net plankton were taken and measured in the laboratory for phytoplankton biomass analysis, cell counting, and identification of bloom-forming species. In the 3 consecutive years of observation, the most frequent bloom-forming species observed were Skeletonema costatum, Pseudonitzschia Spp., and Noctiluca scintillans, The most frequent location of the blooms observed was in the inner part of the bay, in the area in front of Marunda, Ancol and Angke. Phytoplankton biomass during the bloom events were ranged between 126.3 to 232.3 μg Chl-a L−1, with the highest was in 2015 of during the Skeletonema costatum bloom. Blooms were also observed but in low frequency and low biomass at the middle part of the bay off Marunda coastline area. Cell phytoplankton counting were ranged from 1.139 ϗ 106 cell L−1 to 7.893 ϗ 106 cell L−1. During period of surveys, there was no mass mortality of fishes were detected.

Diatom assemblages from sediment traps in response to large seaweed Gracilaria cultivation off Nan'ao island, South China

Most previous studies on seaweed bioremediation largely focus on laboratory control tests or short-time, in-situ field experiments. Here we present a dataset from 4-year-long field monitoring (2014–2018) in a mariculture area of the Baisha Bay off Nan'ao Island, South China, attempting to examine how the large seaweed Gracilaria lemaneiformis cultivation affects a mariculture ecosystem. The temporal variation in seawater physicochemical properties (TN, TP, Reactive silicate, CODMn, and chlorophyll α) and sediment proxies (TN and TP) suggest that the Baisha Bay maintained a better water quality during the Gracilaria cultivation periods (December–May) than other months. Additionally, sedimentary diatom assemblages during the Gracilaria cultivation periods showed an increase in Shannon-Wiener values, but a decrease in the dominant Thalassionema nitzschioides abundance, total diatom abundances and P/(A + B) ratios [Planktonic/(Attached + Benthic)]. These changes consistently suggest that Gracilaria cultivation may have helped improve seawater quality and phytoplankton diversity, thereby maintain a healthy mariculture ecosystem.

Blue carbon stock of the seagrass meadows of Gulf of Mannar and Palk Bay off Coromandel Coast, south India

Blue carbon stock of the seagrass meadows of Gulf of Mannar and Palk Bay, off Coromandel Coast, south India, were computed from the organic carbon content and dry bulk densities of sediment core taken from the seagrass meadows of these two ecosystems. The Gulf of Mannar (GoM) and Palk Bay (PB) harbour 13 seagrass species dominated by Cymodocea serrulata and Syringodium isoetifolium. The soil carbon density of both GoM and PB were higher in subsurface cores. The blue carbon stock of seagrass meadows of the GoM was estimated as 0.001782 Tg and that of PB as 0.043996 Tg. The estimated value of blue carbon stored in seagrass meadows of GoM was 17820 US$ and that of PB was 43,99,682 US$. The results of this study are discussed in the light of climate change mitigation, emphasising the need to conserve these underwater meadows.

Possible fluid discharge associated with faults observed by a high-resolution dense-2D\xa0seismic reflection survey in Uchiura Bay off Numazu, Japan

Although fluid behavior along faults has often been associated with earthquakes and hydrocarbon accumulation, few observations of active, ongoing fluid discharge along the faults exist. We conducted a dense-2D multi-channel seismic (MCS) reflection survey in Uchiura Bay off Numazu, Japan, in 2018 to obtain a high-resolution image of active faults. As a result, active faults cutting the seafloor and possibly active faults cutting reflections immediately below the seafloor have been clearly imaged on MCS profiles. The spatial variation in their displacement on the seafloor has also been quantified. The seafloor displacement of the most obvious active fault, Fault A, changes by more than 2 m within a short horizontal distance of 50 m. On one of the MCS profiles, an enigmatic wavefield (EWF) is observed within the water column above the seafloor near the faults. From careful observations of the raw field data and shot records, the EWF shows three patterns that suggest fluid flow. Based on a velocity analysis of the seawater, the velocity of the EWF zone was somewhat higher than that of its surroundings. Because the seawater velocity is a function of temperature and salinity at the same depth, the effect of temperature is generally greater than that of salinity, so that the higher velocity would suggest a higher temperature. If temperature of the fluid flow is higher than that of its surroundings, the fluid would rise with its buoyancy and spread out along the sea surface. This scenario could well explain the three EWF patterns. From the appearance and non-appearance of the most obvious EWF on adjacent seismic lines, some fluid discharge might have been occurring intermittently from the seafloor near the faults in Uchiura Bay.

Rare great hammerhead predation of a wedgefish

Many species of sharks are apex predators, exerting top-down control on coral reef communities via predation and behavioral modification of prey. The great hammerhead (Sphyrna mokarran) is broadly distributed in tropical waters, with a varied diet composed of crustaceans, cephalopods, bony fish, and other elasmobranchs. Gut-content analysis has indicated a preference for rays and skates, including larger taxa such as guitarfish; however, direct observations of predation are rarely witnessed. At noon on 7 Nov 2019, we observed a large great hammerhead shark (~3.5–4 m total length) preying on a bottlenose wedgefish (Rhynchobatus australiae) (~2–2.5 m total length) at a depth of 2.5 m in a shallow bay off Enderby Island (top photo), in the Dampier Archipelago, along the north coast of Western Australia. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Large-scale helicopter rescue of cruise passengers and freighter crew off the coast of Norway in stormy weather.

During a storm on the 23rd of March 2019, southbound Viking Sky was crossing the notorious Hustadvika bay off Norway with 1373 (915 passengers, 458 crew) aboard when power was lost and the ship drifted towards the shore. Mayday was called at 14:15. When the dropped anchors caught and one engine had started, the ship was about 100 m from the rocky coast. Helicopter evacuation was started at 15:30, but was slightly delayed around 19:00 when 9 crewmembers from a nearby powerless freighter, Hagland Captain, had to be airlifted to safety. The helicopter rescue from Viking Sky was called off at mid-day on the 24th of March. Using its own engines the ship arrived in Molde at 16:20 with 436 passengers and 458 crewmembers. In all, 479 passengers, many of them elderly and three seriously injured, had been airlifted off the ship one-by-one in rough weather by a relay of 6 helicopters, making this one of the most remarkable helicopter rescue operations ever.

A Yesso Scallop Population Exposed to Climate-Induced and Anthropogenic Habitat Changes in Amur Bay, Sea of Japan

In the study area, located in the coastal waters of Amur Bay off Vladivostok, which is exposed to industrial and domestic sewage effluents, the population of the Yesso scallop Mizuhopecten (=Patinopecten) yessoensis and the community of the endo- and epibiosis of its shell underwent major changes during the period 1981–2016, in which three main stages can be identified. During the first stage, which lasted until the early 1990s, the growth rate and lifespan of the scallop decreased, but the mortality rate and the degree of shell bioerosion grew. This stage coincided with the period of increased anthropogenic pollution in Amur Bay. In the mid-1990s, which was a transitional period, the decline in the scallop growth rate ceased. At that time environmental pollution decreased due to reduced sewage discharge as industrial production fell. During the second stage, which lasted until the mid-2000s, the scallop growth rate increased, but the degree of bioerosion and abundance of epibionts on the scallop shell reduced. That was a period when the bay underwent gradual natural self-purification. During the third stage, which continues until present, the scallop growth rate has declined again due to increased eutrophication, which leads to lower oxygen saturation of water. This is most evident during periodic increases in water temperature associated with the quasi-biennial and 7- to 8-year temperature fluctuation cycles. Poaching is a negative anthropogenic that has caused a substantial reduction in scallop population density.

Beach-cast as biofertiliser in the Baltic Sea region-potential limitations due to cadmium-content

Macroalgal mass blooms and accumulating beach-cast are increasing problems in many coastal areas. However, beach-cast is also a potentially valuable marine bioresource, e.g. as a biofertiliser in coastal agriculture. One limiting factor in use of beach-cast as a fertiliser is uncertainty regarding the cadmium (Cd) concentration depending on beach-cast composition and location. In this study, chemical analyses were performed on beach-cast from Burgsviken Bay off Gotland, in the Baltic Sea. The results revealed large variations in cadmium concentration depending on sampling location and beach-cast composition, with levels ranging between 0.13 and 2.2 mg Cd/kg dry matter (DM). Of 15 beach-cast samples analysed, one had a cadmium content above the Swedish statutory limit for sewage sludge biofertiliser (2 mg Cd/kg DM) and four had values above the limit suggested by the Swedish Environmental Protection Agency for 2030 (0.8 mg/kg DM). Species-specific analysis revealed that eelgrass (Zostera marina) contained significantly higher cadmium concentrations than filamentous red algae species (Ceramium and Polysiphonia spp.). Avoiding eelgrass-rich beach-cast by seasonal timing of harvesting and monitoring differences in cadmium concentrations between harvesting sites could thus facilitate use of beach-cast as biofertiliser.

Effects of slaking and degree of compaction on the mechanical properties of mudstones with varying slaking properties

An earthquake in Suruga Bay off Shizuoka Prefecture in 2009 caused an embankment near the Makinohara Service Area of the Tomei Expressway to fail. One of the causes of this failure was the slaking progression of the Neogene Period mudstone in the lower portion of the embankment.The objective of this study is to understand how the degree of slaking affects the compressive and shearing behaviors of crushed mudstone aggregates. A drying-immersion cycle test is proposed; it illuminates how the number of repeated drying and immersion cycles affects the slaking of fine-grained material. Unconfined compression, triaxial compression, and one-dimensional consolidation tests were carried out on crushed mudstone aggregate specimens created by compaction.Consequently, the unconfined compressive strength and maximum deviator stress in the triaxial compression tests on the crushed mudstone aggregates dropped as the grains became finer due to slaking even if the initial density was the same. The compressibility increased and the consolidation yield stress decreased as the grains became finer particles. The degree to which slaking had progressed affected these characteristics as well.

Acoustically invisible feeding blue whales in Northern Icelandic waters.

Fixed passive acoustic monitoring can be used for long-term recording of vocalizing cetaceans. Both presence monitoring and animal density estimation requires the call rates and sound source levels of vocalizations produced by single animals. In this study, blue whale calls were recorded using acoustic bio-logging systems in Skjálfandi Bay off Húsavík, Northeast Iceland, in June 2012. An accelerometer was attached to individual whales to monitor diving behavior. During 21 h recording two individuals, 8 h 45 min and 13 h 2 min, respectively, 105 and 104 lunge feeding events and four calls were recorded. All recorded calls were down-sweep calls ranging from 105 to 48 Hz. The sound duration was 1-2 s. The source level was estimated to be between 158 and 169 dB re 1μPa rms, assuming spherical sound propagation from the possible sound source location to the tag. The observed sound production rates and source levels of individual blue whales during feeding were extremely small compared with those observed previously in breeding grounds. The feeding whales were nearly acoustically invisible. The function of calls during feeding remains unknown.

Epibenthic macrofaunal community response after a mega-earthquake and tsunami in a shallow bay off central-south Chile

On February 27, 2010, the world’s sixth strongest earthquake on record (8.8 Mw) and tsunami hit central Chile. We assess the response of the epibenthic macrofaunal community following this event in Coliumo Bay, one of the areas most affected by this mega-perturbation. The indicators of aggregate and compositional variability show that 3 years after this event, the community appears to have undergone the following dynamics: (1) At an inter-annual timescale, the community (both in density and biomass) shifted through different structures with apparent directionality; (2) Oceanographic and biological seasonality had a strong cyclical influence on the inter-annual community response; (3) There was spatial homogenization of the community over time (i.e., recovery of diversity), probably promoted by the ecological functionality of scavenger species (i.e., crab Cancer coronatus and snail Nassarius spp.) and by the proportional increase in non-dominant species; (4) Bathymetry and bottom dissolved oxygen also played significant roles in the spatial structure of this community; (5) Three years after the perturbation, total density and total community biomass were still considerably below those described under unperturbed conditions, mainly associated with the decrease in density and biomass of dominant species. Therefore, in spite of this apparent community compositional recovery, the aggregate variability currently remains below the levels reported prior to the effect of the mega-earthquake and tsunami. These results provide evidence that supports both the Cross-Scale Resilience Hypothesis and the Response Diversity Hypothesis.

Production and carbonate dynamics of Halimeda incrassata (Ellis) Lamouroux altered by Thalassia testudinum Banks and Soland ex König

Ocean acidification poses a serious threat to a broad suite of calcifying organisms. Scleractinian corals and calcareous algae that occupy shallow, tropical waters are vulnerable to global changes in ocean chemistry because they already are subject to stressful and variable carbon dynamics at the local scale. For example, net heterotrophy increases carbon dioxide concentrations, and pH varies with diurnal fluctuations in photosynthesis and respiration. Few researchers, however, have investigated the possibility that carbon dioxide consumption during photosynthesis by non-calcifying photoautotrophs, such as seagrasses, can ameliorate deleterious effects of ocean acidification on sympatric calcareous algae. Naturally occurring variations in the density of seagrasses and associated calcareous algae provide an ecologically relevant test of the hypothesis that diel fluctuations in water chemistry driven by cycles of photosynthesis and respiration within seagrass beds create microenvironments that enhance macroalgal calcification. In Grape Tree Bay off Little Cayman Island BWI, we quantified net production and characterized calcification for thalli of the calcareous green alga Halimeda incrassata growing within beds of Thalassia testudinum with varying shoot densities. Results indicated that individual H. incrassata thalli were ~6% more calcified in dense seagrass beds. On an areal basis, however, far more calcium carbonate was produced by H. incrassata in areas where seagrasses were less dense due to higher rates of production. In addition, diel pH regimes in vegetated and unvegetated areas within the lagoon were not significantly different, suggesting a high degree of water exchange and mixing throughout the lagoon. These results suggest that, especially in well-mixed lagoons, carbonate production by calcareous algae may be more related to biotic interactions between seagrasses and calcareous algae than to seagrass-mediated changes in local water chemistry.