Benthic Microalgae Research Articles

Warming could shift the phenological responses of benthic microalgae in temperate intertidal zones

Intertidal mudflats colonized by sediment-dwelling microphytobenthos deliver a wide range of ecosystem services. Here we simulate the response of microphytobenthos, located on a temperate tidal mudflat along the French Atlantic coast in Northwestern Europe, exposed to changes in light, temperature, and sea level conditions predicted by the Intergovernmental Panel on Climate Change. Without sea level rise, microphytobenthos benefit from the balancing effect of net primary production fluctuations, experiencing an increase in winter and a decrease in summer. Under the worst emissions scenario, microphytobenthos bloom up to 14 days earlier in spring and 5 days later in fall, thereby extending the low-level microphytobenthos biomass period by an additional 3 weeks in summer. Sea level rise reduces light exposure leading to a pronounced decline in microphytobenthos under the medium-low emissions and worst emissions scenarios. We provide evidence that the anticipated warmer climate and sea level rise will have an impact on microphytobenthos, potentially triggering cascading effects across the entire food web and disrupting ecosystem services.

River-estuary continuum highlighted by variabilities in carbon and nitrogen stable isotope ratios of the catadromous eel Anguilla japonica.

Estuaries exhibit high physicochemical variability and the properties of estuaries and the constituent segments are not yet systematically understood. This study aims to reveal the spatial heterogeneity of predominant organic sources using carbon and nitrogen stable isotope ratios (δ13C and δ15N, respectively) of Japanese eels (Anguilla japonica), one of the ideal natural samplers. In the Miyakoda River, Japan, our study site, the effectiveness of eel δ13C and δ15N values as standard indices of predominant organic sources was tested by employing the River Continuum Concept. This study then extended the application of these indices to the estuary, i.e., the Hamana Lake system, into which the Miyakoda River flows. Once in the upper estuary, eel δ13C and δ15N values became high, with the latter peaking in this river-estuary continuum, indicating that artificial labile subsidies (nutrients and organic matter) with high δ15N values were rapidly assimilated into the food web. Eel δ15N values decreased again in the middle estuary. Nevertheless, the influence of terrestrial organic subsidies extended into this segment, as evidenced by the low δ13C values of eels. These results suggest that refractory organic matter with low δ15N values, such as plant-derived ones, is slowly assimilated into the food web in the downstream estuarine segments. The higher δ13C values in the lower estuary suggested that the contribution of eelgrass or macroalgae occurred in addition to benthic microalgae. Thus, our results emphasize the need to consider the multiple energy flows to understand the estuary as a continuum.

Enhanced growth of benthic microalgae by tablet from liquid dairy cattle manure-based anaerobic digestate

An effective strategy for utilizing anaerobic digestates is required to promote biomass power generation. We developed an anaerobic digestate tablet using liquid dairy cattle manure derived from a small mesophilic anaerobic digester installed on a dairy farm. Anaerobic digestate tablets are intended for use in the fertilization of oligotrophic coastal seas to promote primary production. The purpose of this study was to evaluate (1) the dissolution behavior of nutrients from anaerobic digestate tablets and (2) the effect of the application of anaerobic digestate tablets on the growth of benthic microalgae using a culture experiment. Batch experiments were conducted to investigate the dissolution behavior of the nutrients. Cumulative amounts of dissolved inorganic nitrogen and phosphate in the anaerobic digestate tablet ranged from 110 to 28.9 μg g−1 after 28 days. The dissolved inorganic nitrogen in the anaerobic digestate tablet was mainly ammonium nitrogen and accounted for 92.4–96.9%, which is advantageous for the growth of microalgae. The growth curve of the benthic microalga Nitzchia longissima was monitored using f/2 medium added to the anaerobic digestate tablet. The growth of Nitzchia longissima was two orders of magnitude greater than that of the positive control. The enhanced growth of Nitzchia longissima by the anaerobic digestate tablet was considered a concomitant effect of moderate dissolution of ammonium nitrogen and high affinity for benthic microalgae. In conclusion, the anaerobic digestate tablets prepared in this study have the advantage of supplying nitrogen to benthic microalgae. This study proposes a new method for utilizing anaerobic digestates.

Temporal changes in microalgal biomass and species composition on different plastic polymers in nutrient-enriched microcosm experiments

Marine plastic debris (MPD) is a potential threat to marine ecosystems, but its function as a vector for the transportation of harmful microalgae and its impact on the habitats of other marine organisms are uncertain. To address this gap in knowledge, we performed month-long experiments in 30 L microcosms that contained plates made of six different plastic polymers (polypropylene [PP], low-density polyethylene [LDPE], high-density polyethylene [HDPE], polyvinyl chloride [PVC], polyethylene terephthalate [PET], and polystyrene [PS]), and examined the time course of changes in planktonic and periphytic microalgae. There were no significant differences in the composition of periphytic microalgae or biomass among the different plastic polymers (p > 0.05). Nutrient depletion decreased the abundance of planktonic microalgae, but increased the biomass of attached periphytic microalgae (p < 0.05). In particular, analysis of the plastic plates showed that the abundance of benthic species that are responsible for harmful algal blooms (HABs), such as Amphidinium operculatum and Coolia monotis, significantly increased over time (days 21–28; p < 0.05). Our findings demonstrated that periphyton species, including benthic microalgae that cause HABs, can easily attach to different types of plastic and potentially spread to different regions and negatively impact these ecosystems. These observations have important implications for understanding the potential role of MPD in the spread of microalgae, including HABs, which pose a significant threat to marine ecosystems.

Photosynthetic Characteristics of Benthic Microalgae Measured by HPLC and Diving Pulse Amplitude Modulated (PAM) Fluorometry on the Nakdong River Estuary of the Korean Peninsula

Photosynthetic Characteristics of Benthic Microalgae Measured by HPLC and Diving Pulse Amplitude Modulated (PAM) Fluorometry on the Nakdong River Estuary of the Korean Peninsula

Functional and compositional responses of stream microphytobenthic communities to multiple stressors increase and release in a mesocosm experiment

Field observations form the basis of the majority of studies on microphytobenthic algal communities in freshwater ecosystems. Controlled mesocosm experiments data are comparatively uncommon. The few experimental mesocosm studies that have been conducted provide valuable insights into how multiple stressors affect the community structures and photosynthesis-related traits of benthic microalgae. The recovery process after the stressors have subsided, however, has received less attention in mesocosm studies. To close this gap, here we present the results of a riparian mesocosm experiment designed to investigate the effects of reduced flow velocity, increased salinity and increased temperature on microphytobenthic communities. We used a full factorial design with a semi-randomised distribution of treatments consisting of two levels of each stressor (2 × 2 × 2 treatments), with eight replicates making a total of 64 circular mesocosms, allowing a nuanced examination of their individual and combined influences. We aimed to elucidate the responses of microalgae communities seeded from stream water to the applied environmental stressors. Our results showed significant effects of reduced flow velocity and increased temperature on microphytobenthic communities. Recovery after stressor treatment led to a convergence in community composition, with priority effects (hypothesized to reflect competition for substrate between resident and newly arriving immigrant taxa) slowing down community shifts and biomass increase. Our study contributes to the growing body of literature on the ecological dynamics of microphytobenthos and emphasises the importance of rigorous experiments to validate hypotheses. These results encourage further investigation into the nuanced interactions between microphytobenthos and their environment and shed light on the complexity of ecological responses in benthic systems.

Basin scale variability of Ostreopsis spp. blooms provides evidence of effectiveness of an integrated sampling approach

Ostreopsis spp. blooms have been occurring in the last two decades in the Mediterranean Sea in association with a variety of biotic and abiotic substrata (macroalgae, seagrasses, benthic invertebrates, sand, pebbles and rocks). Cells proliferate attached to the surfaces through mucilaginous trichocysts, which lump together microalgal cells, and can also be found in the plankton and on floating aggregates: such tychoplanktonic behavior makes the quantitative assessment of blooms more difficult than planktonic or benthic ones. Different techniques have been so far applied for quantifying cell abundances of benthic microalgae for research, monitoring and risk assessment purposes. In this context, the Benthic Dinoflagellates Integrator (BEDI), a non-destructive quantification method for benthic dinoflagellate abundances, was developed and tested within the EU ENPI-CBCMED project M3-HABs. This device allows mechanical detachment of cells without collecting the benthic substrate, providing an integrated assessment of both epiphytic and planktonic cells, i.e. of the number of cells potentially made available in the water volume from “resuspension” which could have harmful effects on other organisms (including humans).The present study confirms the effectiveness of the BEDI sampling device across different environments across the Mediterranean Sea and constitutes the first large-scale study of Ostreopsis spp. blooms magnitude in function of different macro- and meso‑habitat features across the basin.

Dynamics of nitrogen genes in intertidal sediments of Darwin Harbour and their connection to N-biogeochemistry

Microbial mediated nitrogen (N) transformation is subject to multiple controlling factors such as prevailing physical and chemical conditions, and little is known about these processes in sediments of wet-dry tropical macrotidal systems such as Darwin Harbour in North Australia. To understand key transformations, we assessed the association between the relative abundance of nitrogen cycling genes with trophic status, sediment partition and benthic nitrogen fluxes in Darwin Harbour. We analysed nitrogen cycling gene abundance using a functional gene microarray and quantitative PCRs targeting the denitrification gene (nosZ) and archaeal ammonia oxidation (AOA.1). We found a significant negative correlation between archaeal ammonia oxidation and silicate flux (P = 0.004), an indicator for diatom and benthic microalgal activity. It is suggested that the degradation of the diatomaceous organic matter generates localised anoxic conditions and inhibition of nitrification. Abundance of the nosZ gene was negatively correlated with nutrient load. The lowest nosZ gene levels were in hyper-eutrophic tidal creeks with anoxic conditions and increased levels of sulphide limiting the coupling of nitrification-denitrification (P = 0.016). Significantly higher levels of nosZ genes were measured in the surface (top 2 cm) compared to bulk sediment (top 10 cm) and there was a positive association with di-nitrogen flux (N2) in surface (P = 0.024) but not bulk sediment. This suggests that denitrifiers are most active in surficial sediment at the sediment-water interface. Elevated levels of nosZ genes also occurred in the sediments of tidal creek mouths and mudflats with these depositional zones combining the diffuse and seaward supply of nitrogen and carbon supporting denitrifiers. N-cycle molecular assays using surface sediments show promise as a rapid monitoring technique for impact assessment and measuring ecosystem function. This is particularly pertinent for tropical macrotidal systems where systematic monitoring is sparse and in many cases challenged by climatic extremes and remoteness.

Food web structures of irrigated rice fields estimated from carbon and nitrogen stable isotopes: Special reference to the role of filamentous green algae as a food resource of aquatic consumers

AbstractRice fields are characterized by high algal production and dense mats of filamentous green algae, and they function as habitats for various aquatic organisms. However, how algal production supports the secondary production of aquatic consumers in rice fields has not been assessed. Herein, aquatic food web structures in two adjacent rice fields in northeastern Japan were examined using carbon and nitrogen stable isotope analyses. Filamentous algae often showed higher δ13C values (mean, −23.1‰ to −21.4‰) than other organic matter sources, including particulate organic matter, benthic microalgae, and soil organic matter (−30.1‰ to −25.4‰). A δ13C‐based isotopic mixing model estimated that among 19 taxa of herbivores and omnivores, most taxa depended on filamentous algae‐derived carbon (ca. &gt;30%). Particularly, Peltodytes intermedius larvae, a specialist feeder for filamentous algae, showed the highest reliance (mean, 66.7%–80.0%). The microcrustacean, Ostracoda sp. also utilized a diet consisting of a substantial proportion of filamentous algae‐derived carbon (57.6%). Seven out of 10 carnivorous taxa showed reliance on filamentous algae‐derived carbon by &gt;30%. Particularly, larvae of beetles (Berosus sp., Enochrus sp., and Hydrochara affinis) showed the highest reliance (40.2%–44.3%). They could assimilate filamentous algae‐based carbon by consuming herbivores, such as P. intermedius. Because fresh filamentous green algae were eaten less by aquatic consumers, the herbivores could consume decomposed filamentous algae rather than fresh ones. Our results suggest that filamentous algae are one of the major organic matter sources supporting secondary production in rice field food webs.

Simulation of benthic microalgae impacts on water quality in shallow water systems, Corsica River, Chesapeake Bay

Eutrophication and hypoxia represent an ever-growing stressor to estuaries and coastal ecosystems due to population growth and climate change. Understanding water quality dynamics in shallow water systems is particularly challenging due to the complex physical and biogeochemical dynamics and interactions among them. Within shallow waters, benthic microalgae can significantly contribute to autotrophic primary production, generate organic matter, increase dissolved oxygen consumption, and alter nutrient fluxes at the sediment–water interface, yet they have received little attention in modeling applications. A state-of-the-art modeling system, the Semi-Implicit Cross-Scale Hydroscience Integrated System Model (SCHISM), coupled with the Integrated Compartment Model (ICM) of water quality and benthic microalgae, has been implemented in the Corsica River estuary, a tributary to Chesapeake Bay, to study benthic microalgal impact on water quality in shallow water systems. The model simulation has revealed a broad impact of benthic microalgae, ranging from sediment–water interface fluxes to water column dynamics, and the effects are observed from near-field to far-field monitoring stations. High-frequency variability and non-linearity dominate benthic microalgal dynamics, sediment oxygen demand, and nutrient fluxes at the sediment–water interface. Resource competition and supply determine the spatial scope of benthic microalgal impacts on far-field stations and the whole estuary system. Our study shows that benthic microalgae are a significant factor in shallow water dynamics that needs adequate attention in future observation and modeling applications.

A fiddler crab reduces plant growth in its expanded range.

Species across the planet are shifting or expanding their ranges because of climate change. These are climate migrants. Although climate migrants are well documented, their impacts on recipient ecosystems are not. Climate migrants that are also ecosystem engineers (species that modify or create habitats) will likely have profound effects on ecosystems. The Atlantic marsh fiddler crab, Minuca pugnax, is a burrowing crab that recently expanded its range into the northeastern United States. In its historical range, M. pugnax enhances the aboveground growth of the cordgrass Spartina alterniflora, a plant critical to marsh persistence. In a control-impact study, however, we found that Spartina aboveground biomass was 40% lower when M. pugnax was present. Thus, the positive effect of M. pugnax on Spartina aboveground biomass flipped to a negative one in its expanded range. Spartina belowground biomass was also 30% lower on average when crabs were present, a finding consistent with what is seen in the historical range. These impacts on Spartina are likely due to burrowing by M. pugnax. Benthic microalgae was, on average, 45% lower when crabs were present. Fiddler crabs eat benthic microalgae, and these results suggest that fiddler crabs can control algal biomass via grazing. Because fiddler crabs reduced the biomass of foundational primary producers in its expanded range, our results imply that M. pugnax can influence other saltmarsh functions such as carbon storage and accretion as they expand north. Most strikingly, our results suggest that as species expand or shift their range with climate change, not only can they have profound impacts in their new ranges but those impacts can be the inverse of what is seen in their historical ranges.

The influence of flow, nutrient enrichment and aquatic macrophytes on benthic microalgal dynamics in a perched temporarily closed estuary

This study investigated the benthic microalgal dynamics in a temporarily closed microtidal estuary situated along a degraded stretch of coastline, where estuaries are impacted by development, flow modification, and nutrient pollution. The near natural, perched uMdlotane Estuary experienced regular high flow conditions that maintained the open mouth state for 40% of the study period. Increased nutrient input originated from agricultural-return flow derived from the extensive catchment sugarcane farming (55.9%). No notable temporal differences were recorded for the benthic microalgal biomass (range: 0–41.3 mg m−2). The benthic diatom diversity and evenness indices ranged between 0.6 and 3.1 H′ and 0.2–1 J′, respectively. Higher benthic microalgal biomass (14.9 ± 15.6 mg m−2) coincided with lower diatom diversity (1.3 H’) in the lower reaches. The habitat stability provided by the rooted Stuckenia pectinata facilitated benthic microalgal accumulation, while the absence of sediment disturbance contributed to the decrease in the benthic diatom diversity. The benthic diatom species Cocconeis lineata, Halamphora acutiuscula, and Hantzschia amphioxys largely dominated the community. Temporal differentiation in benthic diatom composition was driven by salinity, water temperature, phosphate, and photic depth changes. The pollution tolerant species Cyclotella atomus increased in abundance (sub-dominant RA = 5%) in response to increased phosphate availability. The presence of nutrient tolerant taxa serves as an early warning of impending environmental deterioration. Implementation of an adaptive management approach supported by monitoring is required to facilitate timely interventions to prevent the loss of ecosystem functionality induced by nutrient enrichment.

Symbiotic Relationship of Comasterschlegelii (Crinoidea: Comatulidae) and Gymnolophus obscura (Ophiuroidea: Ophiotrichidae) Derived from Stable Isotope and Fatty Acid Analyses.

Coral reef community exhibits high species diversity and a broad range of biological relationships, including widespread symbiosis and complex food utilization patterns. In our study, we investigated the symbiotic relationship between the commonly crinoid host Comaster schlegelii and its ophiuroid obligatory symbiont Gymnolophus obscura. Using a combination of fatty acid biomarkers and stable isotopic compositions, we explored differences in their organic matter utilization strategies and nutritional relationships. The result of stable isotopes revealed that G. obscura had higher δ15N values than its crinoid host. Particulate organic matter and phytoplankton were identified as the primary food sources for both species, however C. schlegelii showed a higher proportional contribution from benthic microalgae. Fatty acid markers showed that C. schlegelii was more dependent on benthic microalgae such as diatoms, and less on debritic organic matter and bacteria than G. obscura. Elevated δ15N values of G. obscura and similar food source contribution rates between the host and symbiont suggest that ophiuroid feeds on materials filtered by crinoids and have similar diet to the host. Our results provide insights into the symbiotic patterns of crinoids and ophiuroids, while also supplying foundational data on how symbiotic reef species select organic matter utilization strategies to adapt to their environment.

Diet composition and feeding strategy of larvae and juveniles of green riffle goby, Stiphodon elegans in Cimaja Estuary, Indonesia

Green riffle goby, Stiphodon elegans, is an amphidromous goby living in Cimaja River, Sukabumi. Postflexion and juvenile stages of this species migrate from the sea to the river estuary for the recruitment process both in the day and nighttime. The presence of prey strongly influences the success of the recruitment process. The present study, therefore, aims to reveal the diet preferences and feeding strategies of postflexion larvae and juveniles of the green riffle goby during the recruitment process. Postflexion larvae and juveniles of S. elegans were collected day and nighttime using a lift net from December 2020 to May 2021 in the Cimaja River estuary. A total of 13,955 larvae and juveniles were captured. For diet analysis, 455 fish larvae and juveniles were analyzed, consisting of 47 larvae (8.9-11.8 BL) and 408 juveniles (11.9-21.8 BL). The diet composition of postflexion larvae comprised five genera from three different classes of benthic microalgae and zoobenthic. In comparison, fish juveniles’ prey composition comprised 15 genera from 5 different classes of benthic microalgae and zoobenthic. The postflexion larva tends to be more specialized than the juvenile in utilizing prey. There is no significant difference in the diet composition and feeding strategy of postflexion larvae and juveniles during the day and nighttime (p>0.05).

A new diatom-based multimetric index to assess lake ecological status

Eutrophication impairs lake ecosystems at a global scale. In this context, as benthic microalgae are well-established warnings for a large range of stressors, particularly nutrient enrichment, the Water Framework Directive required the development of diatom-based methods to monitor lake eutrophication. Here, we present the diatom-based index we developed for French lakes, named IBDL (Indice Biologique Diatomées en Lacs). Data were collected in 93 lakes from 2015 to 2020. A challenge arose from the discontinuous pressure gradient of our dataset, especially the low number of nutrient-impacted lakes. To analyze the data we opted for the so-called “Threshold Indicator Taxa ANalysis” method, which makes it possible to determine a list of “alert taxa.” We obtained a multimetric index based on specific pressure gradients (Kjeldahl nitrogen, suspended matter, biological oxygen demand, and total phosphorous). Considering the European intercalibration process, the very good correlation between IBDL and the common metric (R2 from 0.52 to 0.87 according to the lake alkalinity type) makes us very confident in our ability to match future IBDL quality thresholds with European standards. The IBDL proved at last to be particularly relevant as it has a twofold interest: an excellent relationship with total phosphorus (R2 from 0.63 to 0.83 according to the lake alkalinity type) and a possible application to any lake metatype. Its complementarity with macrophyte-based indices moreover justifies the use of at least two primary producer components for lake ecological status classification.

Microalgae diversity in varying habitat characteristics in Pasuruan and Sidoarjo coastal areas, East Java, Indonesia

Abstract. Mahmudi M, Arsad S, Fitrianesia F, Ramadhan SF, Arif AR, Savitri FR, Dewinta AA, Ongkosongo AD, Musa M, Lusiana ED. 2023. Microalgae diversity in varying habitat characteristics in Pasuruan and Sidoarjo coastal areas, East Java, Indonesia. Biodiversitas 24: 4418-4426. Microalgae are microscopic eukaryotic organisms which live in various types of habitats. The purpose of this study was to identify the types and the abundance of microalgae in several sub-habitats, namely sediments, mangroves, water columns, and artificial substrates; and to analyze the environmental factors that affect the abundance of microalgae. The research was conducted in several sites in the coastal areas in Pasuruan and Sidoarjo, East Java, Indonesia. A quantitative descriptive method was applied with data collection used a purposive sampling method. Samples were collected using net for planktonic microalgae and using sampling plot for epiphytic and benthic microalgae. The microalgae were grouped using NMDS (non-metric multidimensional scaling) and the relationships between microalgae abundance and water quality parameters were analyzed using CCA (canonical correspondence analysis). The results show that Bacillariophyceae, Cyanophyceae, and Chlorophyceae classes were found in all study sites, but Trebouxiophyceae and Dinophyceae were only found in Pasuruan Beach. The highest abundance of microalgae was found in sediment habitat in Wughoyo Beach, Sidoarjo with 706,605 ind. cm-2. The diversity, the evenness, and the dominance index in both coastal areas ranged from 1.43-2.61; 0.71-0.96; and 0.06-0.27, respectively. Similarity analysis using the NMDS showed that there was no similarity among the three sites, suggesting each site had high microalgal variation. The CCA analysis showed that Bacillariophyceae were found in all sites, indicating its high adaptability. The results of this analysis imply that particular habitat has a unique microalgae diversity, thus preserving diverse habitat types is important.

The influence of oyster reefs and surrounding sediments on nitrogen removal – An in-situ study along the East coast of Australia

Oyster reefs play a crucial role in the removal of nitrogen (N) from aquatic systems by facilitating nutrient regeneration and denitrification, both in their tissues and shells and surrounding sediments. However, we still have a limited understanding about the contribution of each component of the reefs (e.g. oysters vs sediments) to N processes, and whether rates are dependent on site-specific characteristics. To address these knowledge gaps, we conducted an experiment across six oyster reefs along 1080 km of the Eastern Australian coast with different sediment characteristics. By using in-situ clear and dark incubation chambers, we assessed how benthic metabolism, nutrient and dinitrogen gas (N2) fluxes varied among the following treatments: ‘oysters’, ‘sediments’, and ‘sediments + oysters’ that were used to represent components of the whole reef habitat (i.e. reef matrix vs surrounding sediments vs the interaction among them, respectively), and sites. We found that during dark conditions and at siltier sites, N2 effluxes from oysters can be up to 23 times higher than sediments, while N2 effluxes from chambers with both sediments and oysters were similar to sediment treatments, and lower than oyster treatments. These results can be explained by sediment processes including nutrient assimilation by benthic microalgae and/or lower nutrient diffusion into interstitial space. Additionally, oyster treatments showed an uptake of nitrate (NO3-) that was likely converted into N2, whereas sediment treatments showed an overall release of NO3-. In dark conditions, ammonium (NH4+) fluxes remained consistent across treatments and sites, indicating that any exports from oyster excretion (in those treatments including oysters) were either counterbalanced by or comparable to exports from sediments. This study provides evidence that the crucial contribution of oyster reefs to N removal is dependent on interactions between reef components and environmental factors.

Distribution, sources and influencing factors of organic carbon in the surface sediments of the coastal tidal flats in Jiangsu Province

Coastal tidal flats are situated in the interaction zone between the ocean and land and are vulnerable to natural changes, human activities, and global changes; these areas serve as an important mixing zone and burial area for carbon and nitrogen storage. Coastal tidal flats contribute significantly to the global carbon cycle due to their high biological productivity, high sedimentation rate, and low decomposition rate. However, there is a lack of research on the sources and influencing factors of organic carbon in surface sediments in the coastal tidal flat area of Jiangsu Province. In this study, fourteen surface sediment samples were collected from the Jiangsu coastal tidal flats, and the distribution of organic carbon was analyzed. The sources and influencing factors of sediment organic carbon were also investigated by analyzing the contents, ratios and stable isotopes of carbon and nitrogen in the sediments. The results indicated that the total organic carbon (TOC) and total nitrogen (TN) contents in the surface sediments of Jiangsu coastal tidal flats ranged from 0.09% to 0.82% and from 0.01% to 0.1%, with mean values of 0.36% and 0.04%, respectively, and that there was a significant positive correlation between TOC and TN. Moreover, the high-value areas were located mostly along the borders or in sections covered in vegetation, whereas the mudflat areas without vegetation had lower values. Considering the status of the Jiangsu coastal zone, the abandoned Yellow River estuary, Yangtze River inlet, marine benthic microalgae, C3 plants and C4 plants were selected as end members of organic carbon concentrations. The contributions of different sources were quantified using a Bayesian mixture model (MixSIAR). Among them, the abandoned Yellow River estuary and Yangtze River inlet contributed 28.1% and 19.3%, respectively, followed by marine benthic microalgae with a contribution of 26%, whereas C3 and C4 plants made limited contributions of 9% and 8.8%, respectively. Additionally, the sediment particle size, hydrodynamics, and tidal flat vegetation such as Spartina alterniflora were the key factors influencing the organic carbon distribution. In general, this study contributes to a better understanding of the biogeochemical processes and sources of organic carbon in coastal tidal flats. It also provides a solid foundation for the creation of carbon sink measures in nearshore tidal flats.

Linkages between pelagic and benthic biota in a deteriorated coastal lake after restoration, Maruit, Egypt

Until the 1960s, Lake Maruit was one of Egypt’s most productive coastal brackish lakes. Continuous polluted discharge from Alexandria city resulted in long-term deterioration. The Egyptian government started a lake restoration program in 2010. Biological linkages between pelagic and benthic communities were assessed in November 2012 using parasitism and predation. This study examined ectoparasites infesting tilapia fish from 300 samples. The platyhelminth ectoparasite, Monogenea, and parasitic-copepod Ergasilus lizae were detected. Platyhelminthes parasitized Oreochromis niloticus and Oreochromis aureus, whereas the crustacean parasitized Coptodon zillii. The parasitic prevalence was low for Cichlidogyrus sp. and Ergasilus lizae. Benthic biotas were similar across basins. Fish abundance does not respond directly to benthic biotic components. Phytoplankton and benthic microalgae were not the main fish diet. Data on Halacaridae and fish clustered, indicating that either Halacaridae responds to their environment like fish or fish prey upon them because of their size. Linear correlations between pelagic, benthic biota, and parasite-infected fish indicate parasites may control their hosts. Some bioindicators indicate that stressed ecosystems differ from unstressed ecosystems. Fish species and biota abundances were low. Inconsistency in the food web and an absence of direct interactions between prey and predators are bioindicators of disturbed ecosystems. The low prevalence of ectoparasites and lack of heterogenous distribution of the various examined biota are bioindicators of habitat rehabilitation. Ongoing biomonitoring to better understand habitat rehabilitation is suggested.

UAV-acquired imagery with photogrammetry provides accurate measures of mudflat elevation gradients and microtopography for investigating microphytobenthos patterning

Intertidal mudflats are highly productive ecosystems where elevation gradients and complex microtopography drive the growth of benthic microalgae (microphytobenthos; MPB) that form the basis of estuarine foodwebs and are crucial for nutrient cycling, shoreline stabilization, and the persistence of marine and coastal species. Mudflat ecosystems are threatened by human activity and natural stressors and thus need to be mapped and monitored. Unoccupied aerial vehicle (UAV) technologies and digital aerial photogrammetry (DAP) have been successfully implemented to study mudflat environments. However, standardizing the UAV flight parameters needed for optimal DAP performance on mudflats remains outstanding. Here, we systematically determined the optimal flight parameters for collection and photogrammetric processing of UAV-acquired data on mudflats by (1) testing across-track overlap (50, 70, and 90%) and flight elevation (73 m and 110 m) parameters, assessing the accuracy of DAP results against reference data from a mobile laser scanner (MLS), and (2) comparing semi-variograms of digital surface models (DSMs) from two UAV flight elevations. We found that all combinations of UAV flight parameters yielded accurate DAP products; flight elevation had a marginal effect on image alignment and had no effect on accuracy, while across-track overlap had no effect on image alignment of DSM of difference (DoD) values. All UAV and MLS point clouds were aligned with and accuracy of < 0.016 m and absolute values of mean DoDs were all sub-millimeter, ranging from 0.0001 ± 0.0322 to 0.0083 ± 0.0270 m. We conclude that conducting UAV surveys at 110 m elevation with 50% across-track image overlap is sufficient for high-accuracy DAP in mudflats. Finally, we tested the utility of such fine-scale topographic data for ecological applications by comparing elevation and topographic position indices (TPI) of DAP-derived DSMs to MPB abundance, measured as chlorophyll a (chl-a), calculated from UAV-acquired NDVI data. We found that elevation and TPI account for 1.6–17% of the variation in chl-a concentration, and that these relationships depend on distance from shore and mudflat morphology. Our findings contribute to standardizing the application of UAV technologies in mudflats and demonstrate the potential of UAV-acquired data for modeling the relationship between microtopography and MPB on ecologically important mudflats.