Short Water Residence Time Research Articles

Ichthyoplankton drift through fishway in large dam: effect of hydrology, seasonal patterns and larvae condition

The seasonal pattern and the composition of downstream drift of fish eggs and larvae through the fish ladder of the Porto Primavera dam, located in the Upper Paraná River, Brazil, were studied. Additionally, the condition of individual larvae with regard to injuries was examined. Samples were collected monthly over three reproductive seasons at upper and lower pools at the twilight (nightfall and dawn). Ichthyoplankton densities were low, especially for eggs, and the downstream passage of fish eggs and larvae through the fish ladder was negatively correlated to water residence time of the reservoir. Highest larvae densities occurred between December and March, with highest densities in February. The ichthyoplankton abundance was similar between sites and times of day, with the predominance of partially damaged larvae (60%). Seventeen taxa were identified, especially non-migratory species. Long-distance migratory species represented less than 1% of the total. Since many migratory species have found favorable habitats for spawning and development in the tributaries of Porto Primavera Reservoir, the dispersion of larvae from these free-flowing tributaries located far from dam through reservoir may not be beneficial for their survival. At the same time, despite the low density and diversity of migratory species larvae drifting downstream from the dam, this fish ladder may have dispersive potential as long as the reservoir displays a shorter water residence time.

Effects of dam construction on arsenic mobility and transport in two large rivers in Tibet, China

Construction of dams on the Singe Tsangpo (ST) and the Yarlung Tsangpo (YT) Rivers, the upper stretch of the Indus and the Brahmaputra Rivers, respectively, are expected to affect material transport. To evaluate the effects of dam construction on arsenic (As) mobility and transport in the ST River and the YT River in Tibet and the downstream river basins, water column and sediment core samples in the Shiquan Reservoir of the ST River and in the Zam Reservoir of the YT River were obtained in August 2017, and January and May 2018, and additionally, at the inflows and outfalls of the reservoirs. The seasonal variation of dissolved As contents in the inflow water of the Zam Reservoir and the Shiquan Reservoir was regulated by the mixing between the low-As river runoff and the high-As hot spring input. Water residence time (WRT) is a key variable regulating the variation of dissolved As contents in reservoirs and outflow waters with time. The absence of the oxic layer at the sediment-water interface reduced the accumulation of As in surface sediments under high-flow conditions. Arsenic mobility in sediment of the two reservoirs was mainly controlled by Mn oxides and organic matter. Reservoirs with long water residence time are more favorable for As retention. Sedimentation was the main mechanism of As retention. The Shiquan Reservoir with a longer WRT of 385 days can effectively retain 55% of the total arsenic load from upstream, while the Zam Reservoir has no effective retention of arsenic due to the very short WRT of 1.1 days. These have important implications on the geochemical and ecological environments of the downstream river basins.

Riverine Mg isotopes response to glacial weathering within the Muztag catchment of the eastern Pamir Plateau

It has been proposed that the riverine Mg isotopes are sensitive to primary mineral dissolution and the formation of secondary minerals and are served as an effective tracer for chemical weathering. Glacial comminution can produce fresh mineral surfaces that provide easily leached cations to the rivers and thus could potentially modify the riverine Mg isotopic behavior. However, the mechanism controlling riverine Mg isotopes under glacial environment is poorly constrained. Here we investigated Mg and Sr isotopic compositions of river and spring waters, bedrocks and sediments within the glacial-covered Muztag catchment in the northeastern Pamir Plateau. The δ26Mg and 87Sr/86Sr values of dissolved loads along the mainstream increased from −1.56‰ and 0.713050 at the glacier margin to −1.12‰ and 0.713855 at the downstream, respectively, both showing strong negative correlation with elevation. The lower δ26Mg but higher molar Ca/Mg and Ca/Na ratios of glacial river waters than those of non-glacial rivers indicate a preferential dissolution of carbonates under glacial environment, further supported by a negative liner correlation between δ26Mg and carbonate weathering proportion. In contrast, the high δ26Mg but low Ca/Mg and Ca/Na ratios at the downstream reflect an increased input of silicate weathering. Saturation state modelling suggests little chance of secondary mineral formation and limited impact on the Mg isotopic behavior in glacial rivers, characterized by low ionic concentrations and short water residence time. These observations, together with complied data from other glacial rivers, suggest preferential carbonate weathering under glacial environment could directly release low Mg isotopes to river water, different from the process of Mg isotopic fractionation by secondary mineral formation. Our findings would provide insight in carbonate weathering regulating riverine Mg isotopic evolution and its potential influence on global carbon cycle, in particular under present warming scenario.

Hurricanes, El Ni\xf1o and harmful algal blooms in two sub-tropical Florida estuaries: Direct and indirect impacts

Future increases in the intensity of hurricanes and El Niño periods predicted by climate change models have focused attention on their role in stimulating harmful algal blooms (HABs). A series of hurricanes that recently impacted Florida (USA) provided a unique opportunity to explore the relationships between hurricanes, El Niño and HABs in two Florida estuaries subject to repeated intense ecosystem disruptive HABs, the Indian River Lagoon and the St. Lucie Estuary. The roles that hurricanes and El Niño play in contributing to HAB events are examined in the context of key structural and functional features of each estuary and their watersheds, including morphology, water residence time and hydrology, such as the influence of Lake Okeechobee discharges into the St. Lucie Estuary. The most direct impact was the increase in rainfall associated with hurricanes and El Niño, resulting in enhanced nutrient loads which drive HABs in the Indian River Lagoon and Lake Okeechobee. Major HABs in Lake Okeechobee also present an indirect threat of freshwater HAB blooms in the St. Lucie Estuary via mandated discharges from the lake into the estuary during high rainfall periods. Conversely, during the absence of HABs in Lake Okeechobee, short water residence times produced by discharges into the St. Lucie Estuary can result in lower bloom intensities.

The transport and mass balance of fallout radionuclides in Brotherswater, Cumbria (UK)

This paper investigates the role of intervening transport processes on lake sediment records of the atmospherically deposited radionuclides 210Pb and 137Cs. Brotherswater is of particular interest to this issue in that its large catchment/lake area ratio and short water residence time are likely to amplify the influence of these processes, both from the catchment and through the water column. Brotherswater is also unique in being the site of two earlier multicore studies that, together with the present study, span a period of 4 decades. Measurements of fallout radionuclides were made on soil cores, suspended sediments and sediment cores, and the results combined with those from earlier studies to construct mass balances for 210Pb and 137Cs in Brotherswater. The results showed that catchment inputs accounted for 63% of 210Pb entering the lake. Further, just 47% of 210Pb entering the water column was delivered to the sediment record. For comparison, in an earlier study at nearby Blelham Tarn with a relatively smaller catchment but longer water residence time it was shown that 47% of 210Pb inputs were delivered via the catchment, 75% of which were delivered to the sediment record. Results from both sites suggest that 210Pb is predominantly transported on fine particulates with a mean particle size of 3–4 μm. Their relatively slow removal from the water column allows them to be transported relatively uniformly throughout the lake and may help account for the fact that simple 210Pb dating models are relatively reliable in spite of the complexities of the transport processes. Mass balance calculations for 137Cs are more complicated because of the variable fallout record. Measurements of 137Cs in the input stream and water column showed that catchment inputs are still significant 30 years after the last significant fallout (Chernobyl). Modelled results showed that catchment inputs delayed the date of peak inputs of weapons test fallout to the lake though by no more than 2 years. Although the results presented here are primarily concerned with fallout radionuclides and their reliability for dating, they also have implications for the use of sediment archives in reconstructing historical records of other atmospherically deposited substances such as trace metals or persistent organic pollutants.

Biogeochemical Dynamics in a Eutrophic Tidal Estuary Revealed by Isotopic Compositions of Multiple Nitrogen Species

AbstractEstuaries sit at the land‐ocean boundary and act as natural reactors for nitrogen (N) transformations among numerous forms. Superimposing onto physical mixing, biological processes lead to nonconservative N behavior in estuaries. Under the coinfluences of biological alteration and multiple end‐members, however, some N species can exhibit apparent conservation. To explore N dynamics and potential mechanisms that modulate the mixing behaviors, we measured concentrations and natural isotopic compositions of multiple N species (nitrate: NO3−, nitrite: NO2−, ammonium: NH4+, and particulate nitrogen) in a typical eutrophic estuary in southern China. Additionally, N uptake and oxidation rates were measured by using isotope labelling techniques to evaluate processes potentially offsetting the conservative mixing of specific N pools. We found that NO3− followed conservative two end‐member mixing with dual isotopes varying in a narrow range (<1–2‰) due to low microbial preferences. Moreover, δ15N‐NO2− revealed a nonconservative pattern with an involvement of multiple end‐members. The dominant N transformation processes shifted downstream. In the upper estuary, ammonia oxidation (~20 μmol L−1 day−1) dominated NH4+ removal and was accompanied by NO2− accumulation. In the middle‐lower estuary, NH4+ uptake became dominant, with phytoplankton showing strong preference for it over NO3− and NO2−. Based on measured NH4+ uptake rates (9.1–12.5 μmol L−1 day−1 in the light and 0.9 μmol L−1 day−1 in the dark), short water residence time (<1 day) was required to maintain the conservative mixing. Coupling N isotopes in multiple nitrogen species with measurements of N uptake and nitrification, we successfully uncovered N dynamics and distinguished biological processes from physical mixing.

Contributions of Atmospheric Deposition to Pb Concentration and Isotopic Composition in Seawater and Particulate Matters in the Gulf of Aqaba, Red Sea.

Lead concentrations [Pb] and isotope ratios (206Pb/207Pb, 208Pb/207Pb) have been measured in samples of total suspended particulate (TSP) aerosols, seawater, and suspended and sinking particles in the Gulf of Aqaba (GOA), Red Sea. Isotope ratios of Pb in seawater and in the soluble fraction of Pb in atmospheric TSP were similar suggesting that TSP is an important source of Pb in this area. Pb concentrations in seawater measured in this study (max 76.8 pmol kg-1) were much lower than those recorded at the same location in 2003-2004 (up to 1000 pmol kg-1). Changes in Pb isotope ratios in TSP depositions in these years indicate that leaded gasoline was responsible for the high dissolved Pb in GOA more than a decade ago and that recent regulation reduced Pb contamination. The similarity in Pb isotope ratios in suspended and sinking particles implies close interactions between these two size fractions. This study demonstrates the effect of the phasing out of leaded gasoline on TSP and seawater Pb chemistry in the Northern GOA; the rate of change in dissolved Pb concentrations in the GOA is faster than that reported for the open ocean possibly due to higher particle scavenging and the relatively short residence time of deep water in the Basin.

Seasonal change of geochemical sources and processes in the Yenisei River: A Sr, Mg and Li isotope study

Abstract The Yenisei River has the highest annual discharge of all the rivers draining into the Arctic Ocean and therefore any change in the chemical flux composition could have a profound effect on the nutrient input to the ocean. The hydrological cycle of this region is predicted to change because of changing precipitation patterns and thawing permafrost. By studying the seasonal changes in the geochemistry of the river, information on which end-member sources are important under different hydrological conditions can be gained and therefore provide a basis to model future change. In this study we present a 7-month time-series of water chemistry samples collected at Igarka near the mouth of the Yenisei. We investigate seasonal variation in major cation chemistry, radiogenic Sr ( 87 Sr / 86 Sr ), stable Li ( δ 7 Li ) and stable Mg ( δ 26 Mg ) isotopes. Temporal variation in major cation chemistry was consistent with seasonal changes in the relative contribution of end-members previously identified from smaller rivers in this region: snow, shallow soil (organic layer and leaf litter) and deeper soil/groundwater. Temporal variation in 87 Sr / 86 Sr values, a source tracer, was also broadly consistent with seasonal changes in the contribution of the aforementioned end-members to river chemistry. However, an additional contribution from atmospheric dust, which is a minor component of major cation fluxes, is required to account for the observed increase in 87 Sr / 86 Sr values during the spring flood. Variations in Li isotopes are observed, with lower values during spring flood. Although there is a relationship between 87 Sr / 86 Sr and δ 7 Li , the variation in δ 7 Li values is unlikely to be due to changes in source inputs. Rather, they are consistent with decreased isotope fractionation during high discharge as a result of the short water residence time inhibiting processes known to fractionate Li isotopes such as adsorption and secondary mineral formation. The seasonal variation in δ 26 Mg values is negligible, but the inter-annual variation observed between 2012 and 2015 may be due to changes in permafrost extent.

Calanoid copepod zooplankton density is positively associated with water residence time across the continental United States

Inherent differences between naturally-formed lakes and human-made reservoirs may play an important role in shaping zooplankton community structure. For example, because many reservoirs are created by impounding and managing lotic systems for specific human purposes, zooplankton communities may be affected by factors that are unique to reservoirs, such as shorter water residence times and a reservoir’s management regime, compared to natural lakes. However, the environmental factors that structure zooplankton communities in natural lakes vs. reservoirs may vary at the continental scale and remain largely unknown. We analyzed data from the 2007 U.S. Environmental Protection Agency’s National Lakes Assessment and the U.S. Army Corps of Engineers’ National Inventory of Dams to compare large-bodied crustacean zooplankton communities (defined here as individuals retained by 0.243 mm mesh size) in natural lakes and reservoirs across the continental U.S. using multiple linear regressions and regression tree analyses. We found that large-bodied crustacean zooplankton density was overall higher in natural lakes compared to reservoirs when the effect of latitude was controlled. The difference between waterbody types was driven by calanoid copepods, which were also more likely to be dominant in the >0.243 mm zooplankton community in natural lakes than in reservoirs. Regression tree analyses revealed that water residence time was not a major driver of calanoid copepod density in natural lakes but was one of the most important drivers of calanoid copepod density in reservoirs, which had on average 0.5-year shorter water residence times than natural lakes. Reservoirs managed for purposes that resulted in shorter residence times (e.g., hydroelectric power) had lower zooplankton densities than reservoirs managed for purposes that resulted in longer residence times (e.g., irrigation). Consequently, our results indicate that water residence time may be an important characteristic driving differing large-bodied zooplankton dynamics between reservoirs and natural lakes.

Regional diversity of complex dissolved organic matter across forested hemiboreal headwater streams

Dissolved organic matter (DOM) from soils enters the aquatic environment via headwater streams. Thereafter, it is gradually transformed, removed by sedimentation, and mineralised. Due to the proximity to the terrestrial source and short water residence time, the extent of transformation is minimal in headwaters. DOM has variable composition across inland waters, but the amount of variability in the terrestrial end member is unknown. This gap in knowledge is crucial considering the potential impact large variability would have on modelling DOM degradation. Here, we used a novel liquid chromatography –mass spectrometry method to characterise DOM in 74 randomly selected, forested headwater streams in an 87,000 km2 region of southeast Sweden. We found a large degree of sample similarity across this region, with Bray-Curtis dissimilarity values averaging 8.4 ± 3.0% (mean ± SD). The identified variability could be reduced to two principle coordinates, correlating to varying groundwater flow-paths and regional mean temperature. Our results indicate that despite reproducible effects of groundwater geochemistry and climate, the composition of DOM is remarkably similar across catchments already as it leaves the terrestrial environment, rather than becoming homogeneous as different headwaters and sub-catchments mix.

Quaternary Landforms and Basin Morphology Control the Natural Eutrophy of Boreal Lakes and Their Sensitivity to Anthropogenic Forcing

Both natural and anthropogenic changes in boreal lakes have been studied utilizing paleolimnological methods, but the spatial variation in the natural conditions of lakes and its connection to geological factors has drawn less attention. Our aims were to examine the spatial distribution of naturally eutrophic lakes on the previously glaciated terrain of central-eastern Finland and the relationship between pre-human disturbance water quality and geological factors related to the basins and their catchments. Furthermore, we studied the pre- to post-human disturbance changes in the diatom assemblages and water quality of 48 lakes (51 sampling sites) across the pre-disturbance phosphorus gradient by using the top-bottom sampling approach and multivariate statistics. According to our results, naturally eutrophic boreal lakes are more common than previously thought, occurring on fine-grained and organic Quaternary landforms, including fine-grained till. Our study emphasizes the importance of the previously overlooked matter of till grain-size variation as a driver behind the spatial variation in the natural trophic states of boreal lakes. The location of a lake in the hydrologic landscape and basin morphology appear to be important factors as well. Shallow, naturally eutrophic lakes with short water residence times and high catchment area to lake area and volume ratios have been particularly sensitive to anthropogenic forcing. Our results indicate that cultural eutrophication is not the only water protection challenge for the relatively remote and dilute boreal lakes, but salinization and alkalinization are also serious threats that should be taken into account. Therefore, it is crucial to consider the notable variation in the natural conditions of boreal lakes in addition to mitigating the effects of anthropogenic forcing, such as nutrient loading, catchment erosion, salt pollution, and climate change, in order to achieve efficient water protection.

Factors behind the variability of phosphorus accumulation in Finnish lakes

Phosphorus retention (TPacc) is one of the major water quality regulators in lakes. The current study aimed at ascertaining the specific lake characteristics regulating TPacc. Moreover, we were interested whether NAO (North Atlantic Oscillation), a proxy of climatic forcing, can explain variability in TPacc, additionally to that ascribed to lake characteristics. Sediment cores were obtained from 21 Finnish lakes, subject to radiometric dating and measurements of TP concentrations. Principal components (PCs) were generated using lake characteristics that are usually included into the modelling of TPacc (e.g. lake area, lake depth, catchment area, P inflow) but also the parameters that the classical models usually missed (e.g. anoxic factor). We used significant principal components (PCs), specific combinations of lake characteristics and monthly NAO values as predictors of TPacc. Lake characteristics explained the bulk of TPacc variability. The most influential factors (positive drivers) behind TPacc included PC1 (representing mainly deep lakes), PC2 (small lakes with high levels of anoxia and water column stability), PC3 (productive lakes with large catchment area and short water residence time), PC4 (lakes with high water column stability, low anoxic factor and relatively high sediment focusing) and PC5 (lakes with high levels of P inflow, anoxia and long water residence time). Additionally, we found a potential negative effect of NAO in October on the annual TPacc. This NAO was significantly positively related to temperatures in surface and near-bottom water layer (also their difference) in autumn, suggesting the possible implications for the internal P dynamics. Increased mineralization of organic matter is the most likely explanation for the reduced TPacc associated with NAO-driven water temperature increase. The analysis presented here contributes to the knowledge of the factors controlling P retention. Moreover, this spatially and temporally comprehensive sediment data can potentially be a valuable source for modelling climate change implications.

Water residence time in the Włocławek dam reservoir (the Vistula river, Poland) affects its macrozoobenthos structure

The aim of our study was to compare the macrozoobenthos structure as well as water and organic matter content of the bottom sediments of two hydrologically different zones of a strongly fluvial the Włocławek Dam Reservoir. Samples were collected from the Włocławek Dam Resevoir at six sites. Three of them were located in the upper, rheolimnic part (URP) of the reservoir and three others in its lower, limnetic part (LLP). Water transparency, sediment water content and organic matter content in the sediments were higher in the LLP. The higher number of taxa and diversity of macrozoobenthos were found in the URP of the reservoir. The density of bottom fauna was slightly higher in the LLP (90,990 ind. m−2) than in the UPR (73,486 ind. m−2), while the significantly higher biomass of macrozoobenthos was found in the URP of the reservoir (2314 g · m−2) than in the LLP (336 g · m−2). The dominant taxa of Oligochaeta and Chironomidae larvae did not show significant differences in the density between both zones. Some taxa were found only in the URP. In this zone, significantly higher densities of other taxa were observed. The only species with greater abundance in the LLP wasPotamothrix hammoniensis(Oligochaeta). Due to the very short water residence time, the differences in the parameters under study between the URP and the LLP of the reservoir were smaller as well as the abundance of the macrozoobenthos was distinctly higher than in other reservoirs.

How length of light exposure shapes the development of riverine algal biomass in temperate rivers?

The impact of cumulative daily solar radiation (CDSR) on the biomass of river phytoplankton (Chl-a) in the growing season was studied using a large dataset of rivers in the Carpathian Basin. The amount of solar radiation was cumulated over the range of 1–60 days. The CDSR–Chl-a relationship could be described by linear regression and appeared to be significant for almost all watercourses with the exception of rivers with short water residence time. To determine the most relevant time period of CDSR impacting phytoplankton biomass, the slopes of regressions were plotted against the accumulating number of days of light exposure (1–60). Two characteristic shapes were obtained: unimodal for rhithral rivers with hard substrate and steady increase for lowland potamal rivers with fine substrate. In both cases, there is an increasing tendency in the slope values with water residence time (WRT). It was demonstrated that CDSR has a pronounced impact on river phytoplankton biomass even in cases when WRT was shorter than the cumulated solar radiation period. These results indicate that development of phytoplankton within the river channel is a complex process in which meroplankton dynamics may have significant impacts. Our results have two implications: First, CDSR cannot be neglected in predictive modelling of riverine phytoplankton biomass. Second, climate models forecast increased drought with subsequently increased CDSR in several regions globally, which may trigger a rise in phytoplankton biomass in light-limited rivers with high nutrient concentrations.

Macrophyte community response to nitrogen loading and thermal stressors in rapidly flushed mesocosm systems

A mesocosm system was developed to simulate estuarine conditions characteristic of short water-residence time ecosystems of the Pacific Coast of North America, and used to evaluate the response of multiple macrophyte metrics to gradients of NO3 loading and temperature. Replicated experiments found that few responses could be directly attributed to NO3 loading up to 6 x ambient. Some response metrics exhibited weak relationships with nutrient loading but could not be resolved with available statistical power. While direct nutrient responses were found for some species-specific metrics (e.g. green macroalgal growth and biomass, tissue N%, etc.), many patterns were confounded with temperature. Temperature generally had a larger effect on response metrics than did nutrient load. Experimental macrophyte communities exhibited community shifts consistent with the predicted effects of nutrient loading at 20°C, but there was no evidence of other eutrophication symptoms (phytoplankton blooms or hypoxia) due to the short system-residence time. The Z. marina Nutrient Pollution Index (NPI) tracked the NO3 gradient at 10°C, but exhibited no response at 20°C, which may limit the utility of this metric in areas with marked thermal seasonality. Results suggest that teasing apart the influence of temperature and nutrients on the expression of eutrophication symptoms will require complex multi-stressor experiments and the use of indicators that are sensitive across a broad range of conditions.

Environmental filters predict the trait composition of fish communities in reservoir cascades

Dam construction alters flow regimes and can change the composition of aquatic communities. Using data from three Brazilian hydrographic basins, we tested the hypothesis that reservoir cascades act as environmental filters for fish traits. This dataset included information on different environmental variables and fish traits (diet, migration, fecundation, parental care, position in the water column, and body size), and we used multivariate analysis (partial RLQ) to quantify the relationships between environmental variables, species abundance and traits. We found that the abundance of migratory species declined towards downstream reservoirs, which tend to be smaller and less turbid with a shorter water residence time than upstream reservoirs. We also found evidence of an association between reservoir age and the domination of fish communities by small-sized species with parental care, external fecundation, and benthic habits. Our findings suggest that particular fish traits are selected for across reservoir cascades.

Complexity in the Biological Recovery of Tatra Mountain Lakes from Acidification

Alpine lakes of the Tatra Mountains were severely affected by acidification, with minimum recorded values of pH ∼4.5 in the mid-1980s. Since the 1990s, a dramatic decrease in the deposition of acidifying compounds has led to a considerable reversal in lake water chemistry (to pH∼5 in the most severely affected lakes). We studied changes of planktonic crustaceans and chironomid occurrence during the acidification period and the following period of recovery from acidification in three categories of 50 Tatra Mountain lakes (non-acidified, acidified and strongly acidified, according to their status at the beginning of the 1980s). In acidified and strongly acidified lakes, the planktonic crustaceans completely disappeared already by about 1976 except for a few individuals of ubiquitous species in littoral zone due to acidification-induced oligotrophication. In strongly acidified lakes, the original planktonic crustaceans disappeared and littoral species became more abundant already before 1976 due to acidification-induced eutrophication and aluminium toxicity. These processes were quickly reversed following the increase in lake water pH. Extinct species started to return to several acidified and strongly acidified lakes already in the beginning of 1990s. The process of recovery was delayed in many other lakes of the same categories, however, or it did not even start before 2008 despite the improved water chemistry and feeding resources (concentration of chlorophyll-a). Compared to planktonic crustaceans, the reaction of chironomids to acidification and recent recovery has been less pronounced. An analysis of sediment records showed that fluctuations in relative abundance of the dominant chironomid taxa and a decrease of their density occurred. In spite of the fact that chironomid fauna exhibited clear signs of recovery in the last two decades, the recovered assemblage does not exactly reflect the pre-acidification status in the lake. The occurrence and higher proportion of more thermophilous chironomid species in some alpine lakes of all categories could be correlated with increasing air temperature. The considerable effect of climatic factors may thus prevent the full re-establishment of the original status even when the acidification stress completely ceases. The delayed return of planktonic crustaceans to some recovered lakes could be a consequence of the short water residence time of these lakes. In addition, a shortening of the water residence time in recent decades, probably related to recent climate change, in interaction with the ecology of planktonic crustaceans, may possibly be causing further delays in their return.

Phytoplankton biomass in a subtropical estuary: drivers, blooms, and ecological functions assessed over space and time using structural equation modeling

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 569:55-75 (2017) - DOI: https://doi.org/10.3354/meps12102 Phytoplankton biomass in a subtropical estuary: drivers, blooms, and ecological functions assessed over space and time using structural equation modeling C. Edward Proffitt* Department of Life Sciences, Texas A&M University - Corpus Christi, Corpus Christi, TX 78412, USA *Corresponding author: ed.proffitt@tamucc.edu ABSTRACT: The Indian River Lagoon (IRL Florida, USA) estuary may be in transition from a benthic productivity state to a phytoplankton-based state because of nutrient inputs, phytoplankton blooms, and light penetration. Variation over both spatial and temporal dimensions reflects a complex ecological topology in the IRL. Understanding features such as phytoplankton blooms and subsequent effects on light penetration, dissolved oxygen, and benthic systems is challenging because of the underlying complexity. Analysis by structural equation modeling can test multivariate hypotheses involving the web of pathways linking biotic and environmental variables. A complex structural equation model composed of linkages among chlorophyll, nutrients, salinity, light penetration, dissolved oxygen (DO), etc. explained phytoplankton biomass (chl a, R2 = 0.46) and relative abundance (chl b and c) and ecological functions (light penetration, R2 = 0.60; DO, R2 = 0.42) across the IRL and over the >20 yr of monitoring data. Results indicate that IRL experiences nitrogen and phosphorus co-limitation of phytoplankton, and that changes in plankton biomass influence ecological functions of DO and light penetration. Partitioning the IRL into regions improved the model fit to observations and showed that different regions of the IRL had different drivers for phytoplankton biomass. Comparing 3 yr groups of data for long and short water residence time regions showed that phytoplankton blooms in the 2010-2012 period (including the ‘superbloom’) were driven primarily by nitrogen and less by phosphorus and that this was associated in part by decomposing phytoplankton producing high concentrations of phosphorus during this period, leading to limitation by nitrogen. KEY WORDS: Phytoplankton · Chlorophyll a · Structural equation modeling · Nutrient · Salinity · Temperature · Indian River Lagoon · Florida Full text in pdf format Supplementary material PreviousNextCite this article as: Proffitt CE (2017) Phytoplankton biomass in a subtropical estuary: drivers, blooms, and ecological functions assessed over space and time using structural equation modeling. Mar Ecol Prog Ser 569:55-75. https://doi.org/10.3354/meps12102 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 569. Online publication date: April 07, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Localised hydrodynamics influence vulnerability of coral communities to environmental disturbances

The movement of water can have a significant influence on the vulnerability of hermatypic corals to environmental disturbances such as cyclone damage, heat stress and anoxia. Here, we explore the relationship between small reef-scale water circulation patterns and measured differences in the abundance, composition and vulnerability of coral assemblages over decades. Changes in coral cover and community structure within Bill’s Bay (Ningaloo Reef, Western Australia) over a 22-yr period, during which multiple disturbance events (including mass bleaching, anoxia, and tropical cyclones) have impacted the area, were compared with spatial variation in water residence times (WRT). We found that reef sites associated with longer water residence times (WRT >15 h) experienced higher rates of coral mortality during acute environmental disturbances compared to reef sites with shorter WRT. Shifts in coral community composition from acroporid to faviid-dominated assemblages were also more prominent at sites with long WRT compared to reef sites with shorter WRT, although shifts in community composition were also observed at sites close to shore. Interestingly, these same long-WRT sites also tended to have the fastest recovery rates so that coral cover was returned to original levels of approximately 20% over two decades. This study provides empirical evidence that spatial patterns in water circulation and flushing can influence the resilience of coral communities, thus identifying areas sensitive to emerging threats associated with global climate change.

Seasonal shifts in the relative importance of local versus upstream sources of phosphorus to individual lakes in a chain

Water quality in the Yahara chain of lakes in southern Wisconsin has been degraded significantly since European settlement of the region, primarily as a result of anthropogenic nutrient inputs. While all four main lakes (Mendota, Monona, Waubesa, and Kegonsa) have undergone eutrophication, elevated phosphorus and chlorophyll concentrations are particularly pronounced in the smaller lakes at the bottom of the chain (Waubesa and Kegonsa). Due to their short water residence times (2–3 months), these lakes are more responsive to seasonal variability in magnitude and source of phosphorus loading compared with the larger upstream lakes. In 2014, more than 80 % of the phosphorus load to Lake Waubesa passed through the outlet of Lake Monona (situated immediately upstream). However, between mid-May and late October when phosphorus concentrations in Lake Monona were reduced as a result of thermal stratification the upstream load dropped to ~40 % of the total, with the majority of loading during this period coming from Lake Waubesa’s local watershed. Correspondingly, seasonal phosphorus trends in Lake Waubesa during summer are correlated with precipitation, rather than phosphorus concentrations leaving upstream lakes. While phosphorus export from the local watersheds of Lakes Waubesa and Kegonsa is relatively small on an annual time scale, targeted loading reductions in these areas during the summertime will most effectively reduce summertime phosphorus concentrations in these fast-flushing lakes. Understanding the interaction of landscape position, water residence time, and mixing regime can help guide watershed management for water quality improvements in lake chains.