Physiological and nutritional constraints on zooplankton productivity due to eutrophication and climate change predicted using a resource-based modeling approach

Emerging evidence suggests that zooplankton production is affected by physiological and nutritional constraints due to climate change and eutrophication, which in turn could have broad implications for food-web dynamics and fisheries production. In this study, we developed a resource-based zooplankton production dynamics model that causally links freshwater cladoceran and copepod daily production-to-biomass (P/B) ratios with water temperature, phytoplankton biomass and community composition, and zooplankton feeding selectivity. This model was used to evaluate constraints on zooplankton growth under four hypothetical scenarios: involving natural plankton community seasonal succession; lake fertilization to enhance fisheries production; eutrophication; and climatic warming. Our novel modeling approach predicts zooplankton production is strongly dependent on seasonal variation in resource availability and quality, which results in more complex zooplankton dynamics than predicted by simpler temperature dependent models. For mesotrophic and hypereutrophic lakes, our study suggests that the ultimate control over zooplankton P/B ratios shifts from physiological control during colder periods to strong resource control during warmer periods. Our resource-based model provided important insights into the nature of biophysical control of zooplankton under a changing climate that has crucial implications for food web energy transfer and fisheries production.

Reef manta rays forage on tidally driven, high density zooplankton patches in Hanifaru Bay, Maldives

Manta rays forage for zooplankton in tropical and subtropical marine environments, which are generally nutrient-poor. Feeding often occurs at predictable locations where these large, mobile cartilaginous fishes congregate to exploit ephemeral productivity hotspots. Investigating the zooplankton dynamics that lead to such feeding aggregations remains a key question for understanding their movement ecology. The aim of this study is to investigate the feeding environment at the largest known aggregation for reef manta rays Mobula alfredi in the world. We sampled zooplankton throughout the tidal cycle, and recorded M. alfredi activity and behaviour, alongside environmental variables at Hanifaru Bay, Maldives. We constructed generalised linear models to investigate possible relationships between zooplankton dynamics, environmental parameters, and how they influenced M. alfredi abundance, behaviour, and foraging strategies. Zooplankton biomass changed rapidly throughout the tidal cycle, and M. alfredi feeding events were significantly related to high zooplankton biomass. Mobula alfredi switched from non-feeding to feeding behaviour at a prey density threshold of 53.7 mg dry mass m−3; more than double the calculated density estimates needed to theoretically meet their metabolic requirements. The highest numbers of M. alfredi observed in Hanifaru Bay corresponded to when they were engaged in feeding behaviour. The community composition of zooplankton was different when M. alfredi was feeding (dominated by copepods and crustaceans) compared to when present but not feeding (more gelatinous species present than in feeding samples). The dominant zooplankton species recorded was Undinula vulgaris. This is a large-bodied calanoid copepod species that blooms in oceanic waters, suggesting offshore influences at the site. Here, we have characterised aspects of the feeding environment for M. alfredi in Hanifaru Bay and identified some of the conditions that may result in large aggregations of this threatened planktivore, and this information can help inform management of this economically important marine protected area.

Effects of a Submesoscale Oceanographic Filament on Zooplankton Dynamics in the Arctic Marginal Ice Zone

Submesoscale structures, characterized by intense vertical and horizontal velocities, potentially play a crucial role in oceanographic dynamics and pelagic fluxes. Due to their small spatial scale and short temporal persistence, conditions for in situ measurements are challenging and thus the role of such structures for zooplankton distribution is still unclear. During RV Polarstern expedition PS107 to Arctic Fram Strait in July/August 2017, a submesoscale filament was detected, which initiated an ad hoc oceanographic and biological sampling campaign. To determine zooplankton taxonomic composition, horizontal and vertical distribution, abundance and biomass, vertical MultiNet hauls (depth intervals: 300–200–100–50–10–0 m) were taken at four stations across the filament. Zooplankton data were evaluated in context with the physical-oceanographic observations of the filament to assess submesoscale physical-biological interactions. Our data show that submesoscale features considerably impact zooplankton dynamics. While structuring the pelagial with distinct zooplankton communities in a vertical as well as horizontal dimension, they accumulate abundance and biomass of epipelagic species at the site of convergence. Further, high-velocity jets associated with such dynamics are possibly of major importance for species allocation and biological connectivity, accelerating for instance processes such as the ‘Atlantification’ of the Arctic. Thus, submesoscale features affect the surrounding ecosystem in multiple ways with consequences for higher trophic levels and biogeochemical cycles.

Evolution of the long-term and estuary-scale phytoplankton patterns in the Scheldt estuary: the disappearance of net growth in the brackish region

Estuaries often show regions in which Chlorophyll-a (Chl-a) accumulates. The location and magnitude corresponding to such accumulation result from a complex interplay between processes such as river flushing, salinity, nutrients, phytoplankton grazing, and the light climate in the water column. Of particular interest is the long-term evolution of the estuary-scale Chl-a distribution in the Scheldt estuary (Belgium/Netherlands) in spring. From 2004–2007, we observed a limited spring-bloom in the brackish region. This bloom intensified in 2008–2014 and disappeared after 2015. This long-term evolution in Chl-a has been linked to simultaneous long-term trends in the suspended particulate matter (SPM) distribution and the improvement of the water quality, which affects grazing of Chl-a by zooplankton. However, this hypothesis has not been systematically investigated. In this paper, we apply two approaches to test this hypothesis. In the first approach, we analyze long-term in situ observations covering the full estuary. These observations include the SPM concentration, zooplankton abundance, and other variables affecting the Chl-a concentration, and show a long-term estuary-scale evolution in not only the SPM distribution but also in zooplankton abundance, freshwater discharge, and maximum photosynthetic rate. In the second approach, we apply a model approach supported by these observations to determine which of the changed conditions may explain the observed change in Chl-a. Our results suggest that a change in SPM alone cannot explain the Chl-a observations. Instead, mortality rate and grazing by zooplankton mainly explains the long-term estuary-scale evolution of Chl-a in spring. Our results highlight that insight into the zooplankton dynamics is essential to understand the phytoplankton (cf. Chl-a) dynamics in the Scheldt estuary.

A report on freshwater tailless flea, Simocephalus vetulus from Haridwar, located in foothills of Shivalik Himalaya in Uttarakhand, India

The Cladocerans, commonly known as “Water fleas” form a primitive freshwater group of micro crustacean zooplankton of the freshwater ecosystem. They play an important role in the aquatic food chain and also contribute significantly to zooplankton dynamics and secondary productivity in freshwater ecosystems. The animals used in the present study were identified as Simocephalus vetulus with the help of identification keys described by various authors in the previous studies from other parts of India. In the present study, the occurrence of “freshwater tailless flea”, S. vetulus (Crustacea- cladocera) is reported for the first time from freshwater bodies in Haridwar, located in foothills of Shivalik Himalayan region in Uttarakhand. The presence of S. vetulus “tailless water flea” will be helpful as a lab model for the health status of aquatic bodies as well as environmental monitoring.