Drift Macroalgae Research Articles

Observing changes of fish community in a semi-arid subtropical coastal lagoon subject to ecological conservation

El Soldado is a semi-arid subtropical coastal lagoon subject to ecological conservation in the Gulf of California, where we reviewed the changes in the fish community structure before (1972; 1978–79; 1990–92) and after (cycle 1, 2018–2019; cycle 2, 2019–2020) the decree as a protected area, and we determined spatio-temporal changes between two consecutive annual cycles. We conducted monthly fish sampling cycles 1 and 2 with a small trawl net at three sites, where water and sediment variables were recorded, and drift macroalgae biomass was estimated. Results showed 99 fish species have been recorded in previous studies, and our study added 11 species. The fish assemblage (2018–2020) comprised 34 marine species and nine of them were dominant in both cycles. In general, higher fish abundance, richness, and diversity were observed in the cycle 1. Spatial changes, in both cycles, showed higher fish abundance, richness, and diversity in the innermost area of the lagoon, characterized by silty sediment and higher drift macroalgae biomass. Temporal changes showed higher fish abundance, richness, and diversity in summer-autumn during cycle 1, while during cycle 2, the higher values were in winter-autumn, mainly associated with seasonal variations in water temperature and the biogeographic affinity of fish species, but were also attributed to changes in the density of species classified as dominant. There was notable a drastic decrease of fish richness, diversity, and density during summer in cycle 2, which was attributed to be a warmer year. El Soldado fish community showed changes before and after the decree as a protected area, but also in the contiguous cycles. These changes include the addition of species, the prevalence of common species over 48 years, and the current dominant species that belong to indicator groups, which exhibit effectiveness in the conservation of this ecosystem.

Spatio-temporal patterns in the biomass, species composition and nitrogen content of drift macroalgae in an urbanised coastal embayment

Understanding spatio-temporal variability in the production of drift macroalgae is useful for monitoring and assessing the influence of local anthropogenic nutrient sources on coastal ecosystems. In this study we: (1) assessed how the drift macroalgal community composition and biomass have changed over a 22-year period in Port Phillip Bay (a large coastal embayment in Victoria, Australia); and (2) measured the amount of accumulated nitrogen in drift macroalgae and the contribution of anthropogenic nitrogen sources through isotopic analysis. The surveys revealed little overlap in species composition (<13%) and a significant decrease in standing stock biomass (from 631 to 49 g m-2) between surveys (1995/1996 and 2017/2018). This was associated with a high degree of turnover (β-diversity) at the genus level between surveys (mean – 0.53), resulting in distinct communities at all sites, particularly among red algal genera. In addition, the occurrence of ephemeral macroalgal blooms and a semi-perennial mat of the red macroalga Botryocladia sonderi at 10-15 m depth at locations close to nutrient sources indicate drift macroalgae are important nutrient accumulators in the bay. Isotopic analysis revealed a high contribution of anthropogenic-derived nitrogen (δ15N > 17‰ and ~ 3% N content) at depths and locations close to nutrient sources, particularly at times of the year when large wastewater discharges occur. Harvesting of drift macroalgae could be a cost-effective solution for offsetting nitrogen inputs from anthropogenic sources, but it will be important to first investigate whether harvesting results in negative impacts to associated fauna or nutrient cycling.

Short-term effect of filamentous macroalgae Chaetomorpha linum on Cymodocea nodosa: Does clonal integration alleviate macroalgae impacts?

Seagrasses are clonal plants that can form meadows in shallow coastal water. Under natural conditions, drift macroalgae can be found associated with seagrass but when facilitated by high nutrient input, opportunistic macroalgae can grow excessively and form mats that impose stressful and highly competitive conditions for seagrasses. In this study, we experimentally investigate the ecological significance of clonal integration in the ability of Cymodocea nodosa to tolerate biotic stress triggered by interactions with the drift macroalgae Chaeotomorpha linum. Our findings provide little support for the hypothesis that clonal integration can influence C. nodosa response to stress, as disconnected plants did not show significant differences in structural and morphological characteristics compared to intact plants. However, the physiological analysis suggests that C. nodosa may still benefit from shared resources with neighbouring plants to mitigate stress caused by C. linum. Moreover, the results indicate that C. nodosa adapts to the presence of the filamentous drift macroalgae C. linum by increasing leaf photosynthetic content, reducing growth rate, and modulating its morphology, regardless of its integration status.

Habitat suitability of drift macroalgae in two shallow coastal estuaries of the northern Gulf of Mexico

Drift macroalgae, often associated with seagrass habitats, may provide resources and habitat to seagrass associated fauna, increasing faunal abundance and diversity. Yet, excessive amounts of drift macroalgae can be harmful, reducing the abundance of both aquatic angiosperms and animals by causing localized hypoxia. To ascertain the effects of drift macroalgae at biomasses commonly found in the northern Gulf of Mexico, we quantified recruitment into drift macroalgae without the presence of seagrass and measured associated changes to temperature, dissolved oxygen, and flow in Redfish Bay, Texas and Big Lagoon, Florida, USA. These locations comprise different coastal ecosystems with different compositions of drift macroalgae. In both locations, the presence of macroalgae significantly increased recruitment of small nekton, with higher abundances and species richness found within macroalgae relative to adjacent artificial macroalgae. Flow velocity and dissolved oxygen levels were lower in macroalgae than in adjacent artificial or bare substrate, but still remained above hypoxic levels over daily fluctuations. Despite lower oxygen levels at night, and overall lower water flow, organisms readily recruited into drift macroalgae more so than the artificial habitats nearby. However, the reduction in oxygen reveals that in higher biomasses, macroalgae could adversely affect animals.

Drift macroalgae positively influence seagrass-associated nekton communities of the northern Gulf of Mexico

Drift macroalgae, often found in clumps or mats adjacent to or within seagrass beds, can provide additional food resources and habitat complexity, leading to increased animal abundance, but large concentrations can also inhibit faunal movements, smother benthic communities, and contribute to hypoxia, reducing nekton abundance. Despite its ubiquity, few studies have quantified drift macroalgal prevalence over large spatial scales or its effects on seagrass-associated nekton, hindering our understanding of the functional role of drift macroalgae in ecosystems. We quantified the relationship between drift macroalgal biomass and the seagrass-associated nekton community within five estuaries spanning 2000 km across the northern Gulf of Mexico. Overall, increases in macroalgal biomass within seagrass meadows significantly influenced community structure, increasing shrimp, crab, and fish abundances, but the effect varied by region. Relationships between species richness, diversity, organism size, and macroalgal biomass were not observed, suggesting that drift macroalgae provide additional habitat but not necessarily new niche space. Small nekton play a vital role in many local fisheries, providing valuable food resources for fish and invertebrates. Increased recruitment into macroalgae can benefit local fisheries by providing shelter and increased food resources, which may increase the survival, growth, and population size of recreationally and economically important species. While excess levels of drift macroalgae can negatively impact benthic plant and animal communities, particularly in eutrophic areas, the moderate levels observed during this survey were associated with positive effects on organismal abundance, suggesting that drift algal dynamics should be considered in habitat-based management strategies for coastal estuaries.

Drift macroalgal distribution in northern Gulf of Mexico seagrass meadows.

Drift macroalgae, often found in clumps or mats adjacent to or within seagrass beds, can increase the value of seagrass beds as habitat for nekton via added food resources and structural complexity. But, as algal biomass increases, it can also decrease light availability, inhibit faunal movements, smother benthic communities, and contribute to hypoxia, all of which can reduce nekton abundance. We quantified the abundance and distribution of drift macroalgae within seagrass meadows dominated by turtle grass Thalassia testudinum across the northern Gulf of Mexico and compared seagrass characteristics to macroalgal biomass and distribution. Drift macroalgae were most abundant in areas with higher seagrass shoot densities and intermediate canopy heights. We did not find significant relationships between algal biomass and point measures of salinity, temperature, or depth. The macroalgal genera Laurencia and Gracilaria were present across the study region, Agardhiella and Digenia were collected in the western Gulf of Mexico, and Acanthophora was collected in the eastern Gulf of Mexico. Our survey revealed drift algae to be abundant and widespread throughout seagrass meadows in the northern Gulf of Mexico, which likely influences the habitat value of seagrass ecosystems.

Spatiotemporal Patterns in the Biomass of Drift Macroalgae in the Indian River Lagoon, Florida, United States

Drift macroalgae plays key roles in the ecology of many coastal systems, including the Indian River Lagoon. In the lagoon, changes in the biomass of drift macroalgae may have interacted with an unprecedented bloom of phytoplankton in 2011. Patterns in the biomass of drift macroalgae were identified using new and original analyses of data from several sampling programs collected between 1997 and 2019. All available data show a relatively low biomass of drift macroalgae in 2010–2012, and surveys of fixed transects and seining as part of a fisheries independent monitoring program also recorded low biomass in 2016. Low light availability and potentially stressful temperatures appeared to be the main influences as indicated by the results of incubations in tanks to determine environmental tolerances and data on ambient conditions. Decreased biomass of drift macroalgae had implications for cycling of nutrients because carbon, nitrogen, and phosphorus not stored in the tissues of drift macroalgae became available for uptake by other primary producers, including phytoplankton. The estimated 14–18% increases in concentrations of these elements in the IRL could have promoted longer and more intense phytoplankton blooms, which would have reduced light availability and increased stress on algae and seagrasses. An improved understanding of such feedback and the ecological roles played by drift macroalgae will support more effective management of nutrient loads and the system by accounting for cycling of nutrients among primary producers.

Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction

At the moment, macroalgae blooms in sea waters, the rotting of which causes greenhouse gas emissions and contributes to the formation of a negative ecological and economic situation in coastal zones, which has become a serious problem. Fuel production through hydrothermal liquefaction (HTL) of macroalgae and marine debris is a promising solution to this ecological problem. The article provides an overview of studies on producing fuel from macroalgae and an assessment of the possibility of their joint recovery with marine debris. The optimal process conditions and their technological efficiency were evaluated. The article shows the feasibility of using heterogeneous catalysis and co-solvent to increase the yield of bio-oil and improve its quality. An assessment of the possibility of joint processing of waste macroalgae and marine debris showed the inexpediency of this direction. The high degree of drift macroalgae contamination also raises the question of the appropriateness of the preliminary extraction of other valuable components for nutrition use, such as fats, proteins, carbohydrates, and their derivatives.

Diversity of drift macroalgae in Aruba, Netherland Antilles, Caribbean Sea, including the rarely reported deep-water species Dasya puertoricensis and Cottoniella filamentosa (Delesseriaceae, Rhodophyta)

Diversity of drift macroalgae in Aruba, Netherland Antilles, Caribbean Sea, including the rarely reported deep-water species Dasya puertoricensis and Cottoniella filamentosa (Delesseriaceae, Rhodophyta)

Dataset on the isotopic (ẟ13C, ẟ15N) and elemental (C, N) composition of estuarine primary producers in the subtropical Southwestern Atlantic coast

Benthic and pelagic primary producers had their isotopic (ẟ13C, ẟ15N) and elemental (C, N) composition monitored in the Patos Lagoons estuary, in southern Brazil. The present dataset comprises temporal data obtained through seasonal samplings of C3 (Scirpus spp.) and C4 (Spartina densiflora) salt marsh plants, ephemerous bloom-forming drift macroalgae (Ulvophyceae), the widgeon grass Ruppia maritima, particulate (POM) and sedimentary (SOM) organic matter in shallow waters (&lt; 2m) of a subtropical estuary from austral summer 2010 to autumn 2016. POM and SOM were collected as proxies of phytoplankton and microphytobenthos, respectively. Salt marsh plants were randomly sampled (N = 126) at a regularly flooded low marsh area, whereas submerged drift macroalgae (N = 29) and Ruppia plants (N = 14) were collected in adjacent mudflats. POM was collected (N = 33) by filtering water samples using glass fiber filter. SOM was obtained (N = 35) by removing superficial sediment. In laboratory, samples were processed and further analyzed for total organic carbon (TOC), total nitrogen (TN) and carbon (13C/12C) and nitrogen (15N/14N) stable isotopes ratios. With a total of 237 samples analyzed, this dataset provides key information on the isotopic and elemental composition of distinct estuarine primary producers and sources of particulate organic matter (POM and SOM) and their temporal variability in a highly variable aquatic environment. Such knowledge may add to ecological studies investigating food webs, biogeochemical cycles and sources tracking in coastal systems.

Incongruent long‐term trends of a marine consumer and primary producers in a habitat affected by nutrient pollution

AbstractNutrient pollution, defined as excess nitrogen and phosphorus related to human activities, is a widespread anthropogenic impact that decreases water quality and limits light penetration in estuarine habitats. These factors can lead to shifts in primary producer communities, but the association between these changes and marine consumer patterns of abundance and growth is relatively unknown. Understanding these relationships is necessary for herbivorous species of conservation concern like the green turtle (Chelonia mydas). We used generalized additive models to analyze concurrent 18‐yr data sets of seagrass cover, drift macroalgae cover, juvenile green turtle abundance, and growth to (1) evaluate long‐term trends in primary producer cover and green turtle demographics; and (2) gain insights into their interrelatedness at a study site known to be affected by nutrient pollution throughout the study period. We found that seagrass cover sharply declined during our study period while macroalgae cover was more resilient with trends varying over time. Juvenile green turtle abundance slowly declined throughout the study period with altered intra‐annual trends, while growth rates remained relatively stable. The incongruence of these trends suggests that relationships between marine consumers and nutrient pollution are more complex than effects on food availability alone. Our results focus on the green turtle but highlight the need for greater understanding of relationships between marine consumer species and nutrient pollution at both local and regional scales.

Combined Effects of Drift Macroalgal Bloom and Warming on Occurrence and Intensity of Diel-Cycling Hypoxia in a Eutrophic Coastal Lagoon

Recently, eutrophication-induced macroalgal bloom and elevating temperature caused by climate change have become major threats to benthic organisms by causing coastal hypoxia. However, combined effects of drift macroalgae and warming on the occurrence and intensity of hypoxia are not well understood, although these anthropogenic stressors have co-occurred. We conducted 10 seasonally replicated 7-day algal enclosure/exclosure experiments at a shallow coastal zone in the eutrophic Nakaumi Lagoon, western Japan, to evaluate the combined effects of drift algae and water warming on occurrence and intensity of diel-cycling hypoxia. Experimental units were 1 × 1 m plots of the sandy bottom fenced within 1 m height with plastic mesh that excluded or included drift algae, with automated dissolved oxygen (DO) sensors deployed at the sediment surface. DO fluctuated over a diel cycle for both algal treatments, and algal presence and elevated temperature additively amplified the diel DO cycle. Algal presence and elevated temperature synergistically increased the occurrence and intensity of diel-cycling hypoxia. The occurrence of hypoxia, including anoxia, increased non-linearly in the presence of algae when mean water temperature exceeded ~ 25 °C, whereas such drastic increase in the hypoxia occurrence was not observed in the absence of algae. Furthermore, the daily minimum DO declined more steeply with warming under algal presence than the absence of algae. These results suggest that coastal areas in the lagoon are now seriously threatened by simultaneous progressions of eutrophication linking to algal bloom and global warming.

Seagrass organic matter transfer in Posidonia oceanica macrophytodetritus accumulations

Seagrass ecosystems are net autotrophic systems which contribute to organic carbon burial in marine sediment. Dead seagrass leaves are often exported outside the seagrass beds and may form accumulations (exported macrophytodetritus accumulations, hereafter EMAs) from littoral zones to deepest canyons. Understanding how seagrass organic matter is channeled in its associated trophic web is necessary to assess the role of the seagrass ecosystem as blue carbon service providers. We used gut content and stable isotope analyses to delineate the Posidonia oceanica EMA food web structure and to determine the importance of detrital material in the diets of macrofauna. Evidence from gut contents and stable isotopes showed that this food web is fuelled mainly by two food sources found in the detritus accumulations: 1) P. oceanica detritus itself and 2) epiphytes and drift macroalgae. Dead leaves of P. oceanica enter the diet of dominant species, representing more than 60% of animal abundance. The food web is structured in five trophic levels with a numerical dominance of detritivore/herbivore species at the first consumer level. Animals act as a vector for seagrass organic matter transfer to upper trophic levels and this “dead seagrass signal” is followed through the entire food web. Seagrass primary production and seagrass organic matter processing by animals are spatially decoupled and this should be taken into account in assessments of seagrass ecosystems as key actors in C cycles in coastal areas.

Reproducible multi-parameter acoustic detection of seasonal drift macroalgae in the Indian River Lagoon, Florida

Biennial single-beam dual-frequency acoustic surveys of Indian River Lagoon, a coastal Florida lagoonal system, were performed to quantify the spatial distribution of the annual peak of seasonal drift macroalgae (DMA) as part of the Surface Water Improvement Program (SWIM) enacted in 1986. The ultimate goal of SWIM was to reclaim historical seagrass acreage. Dense accumulations of DMA can ultimately shade seagrass beds and create anoxic zones via decomposition, especially during warm summer months, thus contributing to the internal nutrient pool. A large portion, approximately 140 km of the Indian and Banana Rivers, were surveyed at 200 m line spacing within the timeframe of peak DMA biomass (April through June). A supervised training catalog of 180 60-s acoustic and video samples was grouped into classes of bare substrate, short (canopy) submerged aquatic vegetation and DMA and segmented by discriminant analysis using a combined 38 and 418 kHz dataset. Percent cover of DMA was binned into categories of 0–33, 33–66 and 66–100%. Consistent classification of training catalog samples collected in 2007, 2008 and 2010 validated the temporal consistency of BioSonics digital transducers, meaning the previous surveys training dataset can be used to produce a preliminary map with only a few days lag-time, and that year-to-year acoustically-derived DMA abundances can be directly compared without bias adjustment. DMA biomass within the 288 km2 survey area was 69,000 and 102,000 metric tons wet weight in 2008 and 2010, respectively with an overall classification accuracy of 78.9 and 83.2%, respectively. Four large-scale deposits were detected in the same locations during both surveys but the majority of biomass came from widespread sparse deposits. DMA was found to decompose within the IRL rather than exiting through Sebastian Inlet. The high abundance of DMA in 2010 and subsequent release of nutrients within the IRL was vital for understanding the 2011 superbloom.

Macroalgal composition and accumulation in New England estuaries

New England estuaries provide essential feeding grounds and nursery habitat for important recreational and commercial species. However, these functions are being altered by a recent shift in estuarine plant dominance from rooted plants to opportunistic drift macroalgae that can form dense accumulations. We hypothesize that formation of these macroalgal accumulations is controlled by the level of nutrient enrichment and the low hydrodynamic energy regime present in many estuarine basins. To test this hypothesis, we conducted temporal macroalgae surveys in eight s.e. Massachusetts estuaries to quantify the level of accumulation within basins with varying levels of nitrogen enrichment and bottom currents. Our results indicate that opportunistic Ulva spp. dominated the macroalgal community in both estuaries with temporal surveys, Green and Great Ponds. Measurements of tidal transport revealed a net import of macrophyte material but with no import or export of Ulva. Within each estuary, occurrence of opportunistic macroalgae was positively related to levels of water column total nitrogen (R2 = 0.76) and growth rate of Ulva spp. directly related to total nitrogen + light level (R2 = 0.92), while bottom coverage was >20% at TN levels >0.48 mgL−1. We conclude that opportunistic species accumulate in response to nutrient enrichment with in situ processes controlling growth and decay, while import and tidal transport play relatively minor roles in the distribution of opportunistic drift macroalgae in these shallow estuaries.

Dynamics of estuarine drift macroalgae: growth cycles and contributions to sediments in shallow areas

Dynamics of estuarine drift macroalgae: growth cycles and contributions to sediments in shallow areas

Acoustic Detection and Mapping of Muck Deposits in the Indian River Lagoon, Florida

ABSTRACT Foster, G.; Riegl, B.M.; Foster, K.A., and Morris, L.J., 2018. Acoustic detection and mapping of muck deposits in the Indian River Lagoon, Florida. Locating significant deposits of muck (semifluid, fine-grained, organic-rich sediment) is an important aspect of estuarine conservation and management. The 38-kHz signal of a dual-frequency, single-beam acoustic survey of drift macroalgae was repurposed post hoc to locate and measure the horizontal and vertical extent of muck deposits within Indian River Lagoon, Florida. Raw echo returns were segmented into 5-cm strata of echo intensity, and a series of postprocessing algorithms were written to identify the characteristic pattern of backscatter associated with muck. Twenty-three deposits thicker than 0.5 m were identified within the 283-km2 survey area, nearly all of which were found within depressions of the Indian River (IR). The quantity of muck was estimated at 1.87 × 106 m3, roughly four times the quantity slated for removal from the Eau Gallie R...

Drift macroalgae in the Patos Lagoon Estuary (southern Brazil): effects of climate, hydrology and wind action on the onset and magnitude of blooms

ABSTRACTEphemeral macroalgal blooms have been increasing in frequency and abundance worldwide. We investigated the variability of drift macroalgae abundance in a shallow bay of the Patos Lagoon Estuary (PLE) in southern Brazil to determine the influence of climate and hydrology on the onset and magnitude of blooms. During the study period (January 2004 to May 2007), three growth cycles were observed and differed in timing, magnitude and persistence. Macroalgal biomass (mostly Ulva species) varied annually and interannually (3 g DW m−2 to 432.7 g DW m−2), peaking during the spring and summer. On an annual scale, macroalgal blooms occurred after reductions in freshwater discharge and associated decreases in water level and increases in salinity and retention. These hydrological parameters were influenced by precipitation which, in turn, is affected by the El Niño Southern Oscillation. On an interannual scale, the largest bloom was observed during a weak-moderate El Niño year. No direct relationship was found between biomass peaks and water nutrient concentrations, but tissue nitrogen content indicated no nitrogen limitation. However, a trend of increasing phosphorus concentration (resulting in decreased DIN:DIP ratios) may be associated with the largest bloom observed. Under favourable hydrological conditions for growth, wind action affected the algal transport in shallow areas, thereby controlling the magnitude and persistence of the blooms. The macroalgal abundance found in the PLE was comparable to that found in temperate eutrophic lagoons, highlighting an algal-dominated period triggered by the combined effects of climate and hydrological factors in a post-El Niño period.

Interaction between Ammonium Toxicity and Green Tide Development Over Seagrass Meadows: A Laboratory Study.

Eutrophication affects seagrasses negatively by increasing light attenuation through stimulation of biomass of fast-growing, bloom-forming algae and because high concentrations of ammonium in the water can be toxic to higher plants. We hypothesized nevertheless, that moderate amounts of nitrophilic macroalgae that coexists with seagrasses under eutrophic conditions, can alleviate the harmful effects of eutrophication on seagrasses by reducing ammonium concentrations in the seawater to non-toxic levels because such algae have a very large capacity to take up inorganic nutrients. We studied therefore how combinations of different ammonium concentrations (0, 25 and 50 μM) and different standing stocks of macroalgae (i.e. 0, 1 and 6 layers of Ulva sp.) affected survival, growth and net production of the seagrass Zostera noltei. In the absence of Ulva sp., increasing ammonium concentrations had a negative influence on the performance of Z. noltei. The presence of Ulva sp. without ammonium supply had a similar, but slightly smaller, negative effect on seagrass fitness due to light attenuation. When ammonium enrichment was combined with presence of Ulva sp., Ulva sp. ameliorated some of negative effects caused by high ammonium availability although Ulva sp. lowered the availability of light. Benthic microalgae, which increased in biomass during the experiment, seemed to play a similar role as Ulva sp.–they contributed to remove ammonium from the water, and thus, aided to keep the ammonium concentrations experienced by Z. noltei at relatively non-toxic levels. Our findings show that moderate amounts of drift macroalgae, eventually combined with increasing stocks of benthic microalgae, may aid seagrasses to alleviate toxic effects of ammonium under eutrophic conditions, which highlights the importance of high functional diversity for ecosystem resistance to anthropogenic disturbance.

The good, the bad and the Ulva: the density dependent role of macroalgal subsidies in influencing diversity and trophic structure of an estuarine community

Worldwide, ecological subsidies enhance ecosystem productivity and therefore trophic support for greater biodiversity of taxa. While studies in terrestrial and aquatic ecosystems demonstrate that the magnitude of subsidies into ecosystems differs widely, the thresholds where subsidies may switch from exerting positive to negative effects are poorly understood. In estuaries, eutrophication promotes drift macroalgae that deposit on the benthos, cover intertidal flats for months and serve as pressed resource subsidies for benthic consumers. We hypothesized there would be a critical threshold of macroalgal biomass where ecosystem‐level effects would turn from positive to negative. We used manipulative field experiments varying macroalgal mat thickness (0.5, 1.5 and 4 cm) over eight weeks and quantified effects on macrofauna on a lagoon mudflat in California. We documented that plots with mat depths of 0.5 and 1.5 cm had higher diversity by supporting both surface feeding and burrowing detritivores. Non‐metric multidimensional scaling showed that the benthic community diverged with mat depth over the course of the experiment. After eight weeks, surface deposit feeders were associated mainly with 0.5 cm macroalgal subsidies, whereas subsurface deposit feeding capitellids were closely linked with 4 cm mats. Depth profiles of pore water sulfide concentration collected from 4 cm mats were 7622 ± 5294 μM, mean ± SE, (mean of means across depth profiles), whereas 0.5 cm treatments resulted in sulfide concentrations that were 0.25% of the 4 cm treatments. This suggests that the mechanism of negative effects for elevated macroalgal subsidies was development of anoxic conditions promoting sulfide accumulation. Thus, our study was the first to find a critical threshold, or ecological tipping point, beyond which the effects of anthropogenically enhanced subsidies to estuarine mudflat communities switched from positive to negative and to describe the mechanism by which elevated subsides altered the abiotic environment and likely reduced ecosystem functioning.