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The Sargassum Watch System processes satellite data and feeds results to a Web portal, giving decision makers timely information on seaweed location and warnings for potential beaching events.

We used laboratory food chains to experimentally explore the independent and combined effects of omnivory and food quality on planktonic trophic interactions. Omnivory is predicted to dampen indirect (trophic cascade) interactions, whereas availability of a low-quality diet, here defined as predator-prey elemental stoichiometric imbalance, should strengthen both direct (grazing and predation) and indirect interactions. Food chains were constructed with a copepod, Acartia tonsa, an aloricate ciliate, Strombidinopsis sp., and two centric diatoms of different size, Thalassiosira weissflogii (larger) and Thalassiosira pseudonana (smaller). Comparing results between food chains with two (copepod and diatoms or ciliate and diatoms) and three (copepod, ciliate, and diatoms) trophic levels tested omnivory effects. Offering grazers phytoplankton grown under nitrogen-replete and nitrogenlimited conditions tested stoichiometric imbalance effects. Carbon ingestion increased by 25% for the copepod and 160% for the ciliate on the nitrogen-limited relative to the nitrogen-replete diatom, demonstrating considerable compensatory feeding by both grazers. On a mixed diet comprised of the nitrogen-limited large diatom and the nitrogen-replete small diatom, the copepod continued to engage in compensatory feeding, but the ciliate did not. Cascading indirect effects of the copepod on phytoplankton via the ciliate were minimal. The decrease in phytoplankton due to direct grazing by the copepod was much greater than any indirect increase resulting from reduced ciliate grazing pressure. Omnivory and compensatory feeding were stronger than positive prey selection for the N-replete intermediate grazer, thus weakening cascading indirect effects of the top predator on phytoplankton.

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Many compelling management issues in Long Island Sound (LIS) focus on how organisms respond to stresses such as commercial and recreational harvesting, eutrophication, hypoxia, habitat degradation, invasion of non-native species, ocean acidification, and climate change. In order to address these complex problems,

The pelagic brown macroalga Sargassum supports rich biological communities in the tropical and subtropical Atlantic region, including a variety of epiphytic invertebrates that grow on the Sargassum itself. The thecate hydroid Aglaophenia latecarinata is commonly found growing on some, but not all, Sargassum forms. In this study, we examined the relationship between A. latecarinata and its pelagic Sargassum substrate across a broad geographic area over the course of 4 years (2015–2018). The distribution of the most common Sargassum forms that we observed (Sargassum fluitans III and S. natans VIII) was consistent with the existence of distinct source regions for each. We found that A. latecarinata hydroids were abundant on both S. natans VIII and S. fluitans III, and also noted a rare observation of A. latecarinata on S. natans I. For the hydroids on S. natans VIII and S. fluitans III, hydroid mitochondrial genotype was strongly correlated with the Sargassum substrate form. We found significant population genetic structure in the hydroids, which was also consistent with the distributional patterns of the Sargassum forms. These results suggest that hydroid settlement on the Sargassum occurs in type-specific Sargassum source regions. Hydroid species identification is challenging and cryptic speciation is common in the Aglaopheniidae. Therefore, to confirm our identification of A. latecarinata, we conducted a phylogenetic analysis that showed that while the genus Aglaophenia was not monophyletic, all A. latecarinata haplotypes associated with pelagic Sargassum belonged to the same clade and were likely the same species as previously published sequences from Florida, Central America, and one location in Brazil (São Sebastião). A nominal A. latecarinata sequence from a second Brazilian location (Alagoas) likely belongs to a different species.

Between 2011 and 2020, 6,790 visual observations of holopelagic Sargassum were recorded across the North Atlantic Ocean to describe regional distribution, presence, and aggregation state at hourly and 10 km scales. Influences of oceanographic region and wind/sea conditions as well as temporal trends were considered; marine megafauna associates documented the ecological value of aggregations. Holopelagic Sargassum was present in 64% of observations from the western North Atlantic. Dispersed holopelagic Sargassum fragments and clumps were found in 97% of positive observations whereas aggregated windrows (37%) and mats (1%) were less common. Most field observations noted holopelagic Sargassum in quantities below the AFAI algorithm detection limit for the MODIS sensor. Aggregation state patterns were similar across regions; windrow proportion increased with higher wind speeds. In 8 of 10 years in the Sargasso Sea holopelagic Sargassum was found in over 65% of observations. In contrast, the Tropical Atlantic and Caribbean Sea exhibited greater inter-annual variability (1–88% and 11–78% presence, respectively) that did not align with extremes in central Atlantic holopelagic Sargassum areal coverage determined from satellite observations. Megafauna association patterns varied by taxonomic group. While some study regions were impacted by holopelagic Sargassum dynamics in the equatorial Atlantic, the Sargasso Sea had consistently high presence and operated independently. Field observations capture important dynamics occurring at fine spatiotemporal scales, including transient aggregation processes and ecological value for megafauna associates, and therefore remain essential to future studies of holopelagic Sargassum.