Kiyoko Yokota

Biotic and abiotic materials attachment to suspended particulate matter in aquatic systems can increase their toxicity and health impacts and has led to an increased need for consistent sampling across various compartments. Sedimentation traps and continuous flow centrifuges are the traditional tools for sampling suspended particulate matter, while manta trawls have been widely used for surface water sampling of suspended or floating microplastics. Limitations, however, exist in the cost of sampling and infrastructure needed to deploy such devices. Here we report on the construction and usage of a novel suspended particulate matter sampling device, the microParticle Obtaining Trap (mPOT). Quality control testing of the mPOT showed suspended particle recovery rates of >90% for particles 100 µm and larger, while field sampling of groundwater, lake and river water shows consistent, size-fractionated recovery of particulate matter. The mPOT is well suited to sample systems not easily accessible by boat or for particles not commonly recovered by common suspended particulate matter sampling and for collection of particles smaller than 300 µm in size.

Cyanobacterial blooms have substantial direct and indirect negative impacts on freshwater ecosystems including releasing toxins, blocking light needed by other organisms, and depleting oxygen. There is growing concern over the potential for climate change to promote cyanobacterial blooms, as the positive effects of increasing lake surface temperature on cyanobacterial growth are well documented in the literature; however, there is increasing evidence that cyanobacterial blooms are also being initiated and persisting in relatively cold‐water temperatures (< 15°C), including ice‐covered conditions. In this work, we provide evidence of freshwater cold‐water cyanobacterial blooms, review abiotic drivers and physiological adaptations leading to these blooms, offer a typology of these lesser‐studied cold‐water cyanobacterial blooms, and discuss their occurrence under changing climate conditions.

The concentration of dissolved organic nitrogen (DON) is increasing in many northern hemisphere lakes, yet its use by phytoplankton and fate in the environment seldom have been quantified. We conducted 1 week, in situ, microcosm incubations across 25 lakes in northeastern North America to understand how DON, dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (P) affected phytoplankton biomass. In addition, we tested whether lakes were limited by single macronutrients (N or P) or colimited by both. Phytoplankton biomass in 80% of lakes responded similarly to DON and DIN additions. Of the lakes where N form produced differential responses, the majority of phytoplankton communities exhibited greater biomass accumulation with DON than DIN. Colimitation was the most common type of nutrient limitation among the study lakes, followed by P limitation. Limitation type shifted with N form in 40% of the study lakes, but without consistent patterns explaining how shifts occurred. Regardless of N form, lakes with watersheds more dominated by agriculture and higher total dissolved nitrogen (TDN) tended to show P‐limited phytoplankton responses, while lakes with less agricultural watersheds and lower TDN tended to show colimited phytoplankton responses. Finally, ambient TDN and total phosphorus (TP) nutrient concentrations were stronger predictors of limitation type than ambient TDN : TP ratios. The different contributions of DON and DIN to phytoplankton biomass in some of our study lakes suggest that DON loading from surrounding watersheds may be an overlooked component in predicting phytoplankton productivity and nutrient limitation dynamics in freshwater ecosystems.

Winter conditions, such as ice cover and snow accumulation, are changing rapidly at northern latitudes and can have important implications for lake processes. For example, snowmelt in the watershed—a defining feature of lake hydrology because it delivers a large portion of annual nutrient inputs—is becoming earlier. Consequently, earlier and a shorter duration of snowmelt are expected to affect annual phytoplankton biomass. To test this hypothesis, we developed an index of runoff timing based on the date when 50% of cumulative runoff between January 1 and May 31 had occurred. The runoff index was computed using stream discharge for inflows, outflows, or for flows from nearby streams for 41 lakes in Europe and North America. The runoff index was then compared with summer chlorophyll‐a (Chl‐a) concentration (a proxy for phytoplankton biomass) across 5–53 years for each lake. Earlier runoff generally corresponded to lower summer Chl‐a. Furthermore, years with earlier runoff also had lower winter/spring runoff magnitude, more protracted runoff, and earlier ice‐out. We examined several lake characteristics that may regulate the strength of the relationship between runoff timing and summer Chl‐a concentrations; however, our tested covariates had little effect on the relationship. Date of ice‐out was not clearly related to summer Chl‐a concentrations. Our results indicate that ongoing changes in winter conditions may have important consequences for summer phytoplankton biomass and production.

The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity^1,2, nutrient biogeochemistry³, greenhouse gas emissions⁴, and the quality of drinking water⁵. The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity^6,7, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification^8,9 or oxygen may increase as a result of enhanced primary production¹⁰. Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world’s oceans^6,7 and could threaten essential lake ecosystem services^2,3,5,11.

Microplastics are an emerging environmental pollutant, whose global ubiquity is becoming increasingly evident. Conventional wastewater treatment does not completely remove them, and there are growing concerns about microplastics in source water and post-treatment drinking water. Microplastics have been reported to alter the development, physiology, and behavior of various aquatic organisms; however, limited knowledge exists on their effect on natural phytoplankton communities. Many studies also use uniformly spherical plastic beads, while most scrub particles in consumer products and secondary microplastics in the environment have various shapes and sizes. We tested the effects of two types of microplastics, 50 µm polystyrene (PS) calibration beads and polylactic acid (PLA) plastic body wash scrub particles, and one type of plant-derived body wash scrub particle on a natural phytoplankton assemblage through a 7-day incubation experiment in a temperate, mesotrophic lake. The calibration beads and the plant-derived particles generally did not alter the taxonomic composition of the phytoplankton in the mesocosms, while the PLA body wash microplastics eliminated cryptophytes (p < 0.001) and increased chrysophytes (p = 0.041). Our findings demonstrate differential effects of irregularly shaped PLA body wash microplastics vs. PS calibration beads on lake phytoplankters and empirically support potential bottom-up alteration of the aquatic food web by secondary microplastics.

In the mid-2000s a survey was conducted to evaluate fish mercury in lakes across New York State. Approximately 10 years later a second survey examining adult sportfish from 103 lakes and reservoirs was conducted to evaluate the response of fish mercury to recent declines in US mercury emissions. Of those lakes, 43 were part of the earlier survey and were examined to determine if mercury concentrations in four popular sport species, Yellow Perch, Walleye, and Small- and Largemouth Bass, declined in response to decreasing emissions. Water samples were also collected at 35 of these lakes and analyzed for mercury, methylmercury and other analytes. The Adirondack and Catskill regions remain biological mercury hotspots with elevated concentrations in fish. The most widely sampled species, Yellow Perch, showed significant increases in mercury in the Northeast and West regions of New York State over the past decade. The increases in Yellow Perch mercury is not consistent with significant reductions in water concentrations of both total and methylmercury observed corresponding in lake water samples. This discrepancy suggests watershed and in-lake processes beyond mercury emissions, such as recovery from acid deposition, impacts from climate change, or changes in food web structure may be controlling fish mercury concentrations. These results demonstrate a need for a consistent, long-term program to monitor fish mercury to inform the status of mercury contamination in New York State.

The limiting nutrient for freshwater phytoplankton productivity can vary within and between geographic regions. Understanding how local (i.e., lake and catchment) and regional (i.e., multiple catchment) factors shift the relative importance of nitrogen (N) and phosphorus (P) limitation presents a key research challenge for freshwater ecosystems and may enhance our understanding of how lakes could be managed to control eutrophication. Here, we used in situ microcosm incubations with factorial N and P amendments to determine the type of limitation in 16 lakes across the northeastern United States. Study lakes had similar climate but varied in geography, trophic status, and chemistry. Limitation by both N and P was most common (colimitation n = 5 and serial limitation n = 3); however, we also observed single nutrient N (n = 3) and P limitation (n = 4). The type of limitation was related to background phytoplankton biomass, longitude, and land use. The magnitude of response to combined nutrient enrichment was negatively related to background P concentrations. This multi-lake experiment suggests that both N and P often play a role in regulating productivity and that local and regional characteristics affect nutrient limitation patterns.