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1. The interaction of physical and biological factors drives phytoplankton spatial distribution in the northern California Current

   https://doi.org/10.1002/lno.11431  

   Krause, Jeffrey W. ; Brzezinski, Mark A. ; Largier, John L. ; McNair, Heather M. ; Maniscalco, Michael ; Bidle, Kay D. ; Allen, Andrew E. ; Thamatrakoln, Kimberlee ( February 2020 , Limnology and Oceanography)

   Transitions in phytoplankton community composition are typically attributed to ecological succession even in physically dynamic upwelling systems like the California Current Ecosystem (CCE). An expected succession from a high‐chlorophyll (~ 10μg L⁻¹) diatom‐dominated assemblage to a low‐chlorophyll (< 1.0μg L⁻¹) non‐diatom dominated assemblage was observed during a 2013 summer upwelling event in the CCE. Using an interdisciplinary field‐based space‐for‐time approach leveraging both biogeochemical rate measurements and metatranscriptomics, we suggest that this successional pattern was driven primarily by physical processes. An annually recurring mesoscale eddy‐like feature transported significant quantities of high‐phytoplankton‐biomass coastal water offshore. Chlorophyll was diluted during transport, but diatom contributions to phytoplankton biomass and activity (49–62% observed) did not decline to the extent predicted by dilution (18–24% predicted). Under the space‐for‐time assumption, these trends infer diatom biomass and activity and were stimulated during transport. This is hypothesized to result from decreased contact rates with mortality agents (e.g., viruses) and release from nutrient limitation (confirmed by rate data nearshore), as predicted by the Disturbance‐Recovery hypothesis of phytoplankton bloom formation. Thus, the end point taxonomic composition and activity of the phytoplankton assemblage being transported by the eddy‐like feature were driven by physical processes (mixing) affecting physiological (release from nutrient limitation, increased growth) and ecological (reduced mortality) factors that favored the persistence of the nearshore diatoms during transit. The observed connection between high‐diatom‐biomass coastal waters and non‐diatom‐dominated offshore waters supports the proposed mechanisms for this recurring eddy‐like feature moving seed populations of coastal phytoplankton offshore and thereby sustaining their activity.

    

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2. Effects of climate change and episodic heat events on cyanobacteria in a eutrophic polymictic lake.

   https://doi.org/10.1016/j.scitotenv.2019.07.220  

   Bartosiewicz, Maciej ( January 2019 , Science of the total environment)

   null (Ed.)

   Changes in mixing regimes and CO2 availability may promote harmful cyanobacterial blooms in polymictic lakes and ponds globally, but the underlying mechanisms still remain unclear. We integrated results from a natural experiment comprising an average-wet year (2011) and one with heat waves (2012), a long-term meteorological dataset (1960–2010), historical phosphorus concentrations and corresponding sedimentary pigment records, to determine, on different temporal scales, the mechanistic controls of cyanobacterial blooms in a eutrophic polymictic lake. Intense warming in 2012 was associated with: 1) increased stability of the water column with buoyancy frequencies exceeding 40 cph at the surface, 2) high phytoplankton biomass in spring (up to 125 mg WW L-1), 3) reduced downward transport of heat and 4) persistently depleted epilimnetic CO2 concentrations. CO2 depletion was effectively maintained by intense uptake by phytoplankton (influx up to 30 mmol m-2 d-1) in combination with reduced carbon inputs from the watershed during dry periods. Under eutrophic conditions these effects triggered massive bloom of buoyant cyanobacteria (up to 300 mg WW L-1). Complementary evidence from polynomial regression modelling using long-term datasets revealed that warming is the most important predictor of cyanobacterial abundance during the second half of the last century explaining 78% of the observed positive trend, whereas phosphorus concentration explained only 10% thereof. Together the results from the interannual comparison and the multi-decadal record indicate that hotter and drier climates increase water column stratification and decrease CO2 availability in eutrophic polymictic lakes. This combination catalyzes blooms of buoyant cyanobacteria. 

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3. Phytoplankton community response to a drought‐to‐wet climate transition in a subtropical estuary

   https://doi.org/10.1002/lno.12348  

   Douglas, Sarah ; Xue, Jianhong ; Hardison, Amber ; Liu, Zhanfei ( April 2023 , Limnology and Oceanography)

   Arid subtropical climates often oscillate between drought and wet conditions, leading to a “flood or famine” paradigm for estuarine freshwater inflow, in which sporadic storm events drive dynamic changes in salinity and nutrient availability. Transitioning from prolonged drought to wet conditions can impact phytoplankton communities. The Mission‐Aransas Estuary, located on the south Texas coast, transitioned from a 5‐yr drought (2010–2015) to wet conditions (2015–2020), punctuated by several large flood events and the direct impact of category 4 Hurricane Harvey. Using an 8‐yr bimonthly sample set (2012–2019), we evaluated particulate organic carbon, chlorophylla, nutrient concentrations, and accessory pigments to characterize the response of the phytoplankton community to these climate events. We found that phytoplankton biomass was diminished during severe drought and increased during prolonged wet conditions. The phytoplankton community switched from being diatom‐dominated during drought to cyanobacteria‐dominated following estuarine freshening, driven by lower salinity and increased nutrient availability. Seasonal fluctuations between taxa persisted regardless of climate condition. The drought‐to‐wet transition prompted a regime shift of the estuarine phytoplankton community to a new quasi‐steady state in the studied estuary. Globally, changing climate regimes may cause longer periods of extreme drought or wet conditions for estuarine systems. Detailed, long‐term ecosystem monitoring is necessary to fully evaluate ecological responses to extreme weather events, especially links between biogeochemical cycling and ecosystem function. These results suggest that oscillations between distinct wet and dry periods have lasting effects on primary productivity, phytoplankton community composition, and organic matter cycling in subtropical estuaries with long residence times.

    

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4. Experimental N and P additions alter stream macroinvertebrate community composition via taxon‐level responses to shifts in detrital resource stoichiometry

   https://doi.org/10.1111/1365-2435.13289  

   Demi, Lee M. ; Benstead, Jonathan P. ; Rosemond, Amy D. ; Maerz, John C. ; El‐Sabaawi, ed., Rana ( February 2019 , Functional Ecology)

   Increases in nitrogen (N) and phosphorus (P) availability are changing animal communities, partly by altering stoichiometric imbalances between consumers and their food. Testing relationships between resource stoichiometry and consumer assemblage structure requires ecosystem‐level manipulations that have been lacking to date.

   We analysed patterns of macroinvertebrate community composition in five detritus‐based headwater streams subject to experimental whole‐stream N and P additions that spanned a steep gradient in dissolved N:P ratio (2:1, 8:1, 16:1, 32:1, 128:1) over 2 years, following a 1‐year pre‐treatment period.

   We predicted that shifts in leaf litter stoichiometry would drive overall patterns of community composition via greater responses of shredders to enrichment than other taxa, as shredders dominate primary consumer biomass and experience larger consumer–resource elemental imbalances than other functional groups in stream ecosystems. Specifically, we expected litter C:P to be a significant predictor of shredder biomass given the greater relative imbalances between shredder and litter C:P than C:N. Finally, we tested whether shredder responses to enrichment were related to other taxon‐level traits, including body size and stoichiometry, larval life span and growth rate.

   Whole‐community composition shifted similarly across the five streams after enrichment, largely driven by increased shredder and predator biomass. These shifts were limited to the autumn/winter seasons and related to decreased leaf litter C:P, highlighting important links between the quality of seasonal litter subsidies and community phenology.

   Among 10 taxa that drove structural shifts, two declined while other taxa from the same functional/taxonomic groups responded positively, suggesting that specific life‐history traits may determine sensitivity to enrichment.

   Increases in total shredder biomass, and in biomass of several common shredders, were associated with lower litter C:P. Body C:P did not predict shredder response to enrichment. However, weak negative relationships between shredder response and body size, and larval life span, suggest that small‐bodied and short‐lived taxa may be more responsive to shifting resource stoichiometry.

   Moderate anthropogenic increases in N and P availability affect resource stoichiometry and can alter animal communities, influencing additional food web and ecosystem properties. We provide support for ecological stoichiometry as a framework for predicting such outcomes based on changes in the elemental composition of resource pools.

   Aplain language summaryis available for this article.

    

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5. Responses of benthic algae to nutrient enrichment in a shallow lake: Linking community production, biomass, and composition

   https://doi.org/10.1111/fwb.13375  

   McCormick, Amanda R. ; Phillips, Joseph S. ; Ives, Anthony R. ( July 2019 , Freshwater Biology)

   Understanding how nutrient limitation affects algal biomass and production is a long‐standing interest in aquatic ecology. Nutrients can influence these whole‐community characteristics through several mechanisms, including shifting community composition. Therefore, incorporating the joint responses of biomass, taxonomic composition, and production of algal communities, and relationships among them, is important for understanding effects of nutrient enrichment.

   In shallow subarctic Lake Mývatn, Iceland, benthic algae compose a majority of whole‐lake primary production, support high secondary production, and influence nutrient cycling. Given the importance of these ecosystem processes, the factors that limit benthic algae have a large effect on the function and dynamics of the Mývatn system.

   In a 33‐day nutrient enrichment experiment conducted in Lake Mývatn, we measured the joint responses of benthic algal biomass, primary production, and composition to nitrogen (N) and phosphorus (P) supplementation. We enriched N and P using nutrient‐diffusing agar overlain by sediment, with three levels of N and P that were crossed in a factorial design.

   We found little evidence of community‐wide nutrient limitation, as chlorophyll‐aconcentrations showed a negligible response to nutrients. Gross primary production (GPP) was unaffected by P and inhibited by N enrichment after 10 days, although the inhibitory effect of N diminished by day 33.

   In contrast to biomass and primary production, community composition was strongly affected by N and marginally affected by P, with some algal groups increasing and others decreasing with enrichment. The taxa with the most negative and positive responses to N enrichment were Fragilariaceae andScenedesmus, respectively.

   The abundances of particular algal groups, based on standardised cell counts, were related toGPPmeasured at the end of the experiment.Oocystiswas negatively associated withGPPbut was unaffected by N or P, while Fragilariaceae andScenedesmuswere positively associated withGPPbut had opposite responses to N. As a result, nutrient‐induced compositional shifts did not alterGPP.

   Overall, our results show that nutrient enrichment can have large effects on algal community composition while having little effect on total biomass and primary production. Our study suggests that nutrient‐driven compositional shifts may not alter the overall ecological function of algal communities if (1) taxa have contrasting responses to nutrient enrichment but have similar effects on ecological processes, and/or (2) taxa that have strong influences on ecological function are not strongly affected by nutrients.

    

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