太湖长兜港固体悬浮物再悬浮的室内模拟

定量研究浅水湖泊动力扰动与沉积物再悬浮的关系,目前仍是湖泊研究中的难点之一。实验利用再悬浮发生装置,对太湖长兜港沉积物风浪扰动进行室内模拟,建立扰动强度与有效波高之间的关系,定量分析风浪对太湖水体中悬浮物的影响及固体悬浮物在水体中的垂向分布,初步得出风浪对太湖沉积物扰动深度一般在毫米级的结论。     

Impacts of wind waves on sediment transport in a large,shallow lake

Lake Okeechobee is a large, shallow subtropical lake, located in south Florida. Over the last several decades, Lake Okeechobee has experienced accelerated eutrophication due to excessive phosphorus loads from agricultural run‐off. Recycling of phosphorus from bottom sediments through resuspension is critical to addressing eutrophication of the lake and for water quality management. The present study investigates the impacts of wind waves on sediment transport in Lake Okeechobee, using measured data and the Lake Okeechobee Environmental Model (LOEM). The LOEM was fully calibrated and verified with more than 10 years of measured data in previous studies. Analysis of the measured data indicates significant wave height (SWH) and suspended sediment concentration are closely correlated to the wind speed in the lake. The nonlinear interaction of high‐frequency wind waves with relatively low‐frequency currents in the boundary layer plays a key role in sediment deposition/resuspension. Without considering the effects of wind waves, the bottom shear stress can be greatly underestimated. The spatial variations of key variables for sediment modelling, including SWH, water depth, orbital velocity, current velocity, bottom shear stress and sediment concentration, are discussed. In general, the near‐bottom wave velocity (and the associated bottom shear stress) is greater than or the same order of magnitude as the near‐bed current velocity (and the associated bottom shear stress) in this shallow water system. Although the sediment zones of Lake Okeechobee were described in previous studies, few published papers discussed its formation mechanisms. The findings of the present study include that the multiyear averaged bottom shear stress with wind‐wave effect plays a key role in forming the spatial patterns of the sediment zones. The study results are currently being used in lake management and in developing strategies for reducing phosphorus in the lake.     

Lake Turkana,major Omo River developments,associated hydrological cycle change and consequent lake physical and ecological change

This study aimed to explore Lake Turkana's ecological reliance on hydrology and to determine the hydrological changes and consequences arising from the major hydropower and irrigation developments in the lake's basin. The major developments on Ethiopia's Omo River are especially significant as this river provides over 80% of the lake's annual freshwater influx and associated nutrients. The cascade of hydropower dams permanently dampens the natural hydrological cycles and lake level variability. The driving force of the flood influx to the lake is curtailed and the pattern of lake currents will adjust. Ultimately 80% of the river inflow to the lake will be regulated. Large volumes of water are required to initially fill the hydropower dam reservoirs. During 2015–16 when the huge Gibe III reservoir was filled, Lake Turkana's water level declined 2?m.The study has shown that large-scale irrigation schemes in the Lower Omo can potentially abstract 50% of the Omo River water, and that this would cause the lake level to shrink permanently to the detriment of the lake ecology. Possible lake level drops of over 15?m are demonstrated. The basin's natural capital is being replaced by large-scale plantation developments. The hydrological changes are drastic and the ecological consequences on Lake Turkana have not been fully understood. Without serious mitigation measures, Lake Turkana is a potential African Aral Sea disaster in the making, emulating what has happened to other great lakes such as Lake Chad.     

EXPERIMENTAL STUDY ON SEDIMENT RESUSPENSION IN TAIHU LAKE UNDER DIFFERENT HYDRODYNAMIC DISTURBANCES

Contaminants resuspension in sediments induced by wind-wave could influence the water quality in shallow lakes. Resuspension of surface sediments from the Zhushan Bay, Taihu Lake was simulated under different wind forcing by using a pneumatic annular flume in this study. Acoustic Doppler Velocimeter (ADV) was used to measure flow velocity at each wind speed, and the characteristics of sediment resuspension were studied with the layered sampling technology. The experimental results show that the flow velocity increases with wind speed obviously and 6m/s is a critical wind speed which affected hydrodynamic conditions significantly. The distribution of flow velocity and water depth is different from that in ordinary open channel. With the enhanced hydrodynamic factors, the Suspended Solids Concentration (SSC) in water increases accordingly, and the incipient velocity of sediment resuspension is about 0.21 m/s. Based on the analysis of wind speed and average SSC in water column, the quantitative relationship is obtained. The SSC of the bottom layer is higher than the content of surface layer under different hydrodynamic conditions, and there are similar distributions between SSC and flow velocity in different water layers.     

Sources and scales of near-bottom turbulent mixing in large meromictic Lake Iseo

Mixing dynamics in the bottom boundary layer (BBL) of lakes is of primary importance for mediating mass and heat fluxes across the upper sediment. In lakes with depth of several hundred meters, the BBL mixing is often suggested to be low; however, quantitative information from these depths is extremely rare. We assessed the mixing conditions in the BBL of Lake Iseo, a 256 m deep, meromictic Italian lake, where anoxia and accumulation of phosphorus is a major issue. High-resolution temperature and currents measurements demonstrated regular development of turbulence at 220 m depth, with dissipation rates of the turbulent kinetic energy up to 10⁻⁷ W kg⁻¹, characteristic of a shear boundary layer. Analysis of temperature and oxygen dynamics revealed a direct link between the turbulence intensification in the BBL and the passage of a 90-h period basin-scale internal wave. This slow oscillatory motion was attributed to lake-wide internal waves of second vertical mode, whose effect in the upper part of the water column was much less profound. The wave passages were able to increase hypolimnetic velocities above 1 cm s⁻¹, to produce thermal instability across the BBL and to significantly enhance turbulent mixing in the deepest waters. During periods of high velocity bursts, the chemical stratification in the BBL was effectively eroded while direct sediment resuspension was unlikely. The new results reveal the turbulent character of the bottom boundary mixing in deep lakes, highlighting the direct link to wind-driven motions and important effects on the water–sediment exchange of solutes.     

Sediment dynamics in shallow Lake Markermeer, The Netherlands: field/laboratory surveys and first results for a 3-D suspended solids model

In 2007/08, a study was undertaken on sediment dynamics in shallow Lake Markermeer, The Netherlands. Firstly, the sediment characteristics median grain size, mud content and loss on ignition showed a spatial as well as water depth related pattern indicating wind-induced sediment transport. Sediment dynamics were investigated in a sediment trap field survey at two stations. Sediment yields, virtually all coming from sediment resuspension, were significantly correlated with wind speeds. Resuspension rates for Lake Markermeer were very high, viz. ca. 1,000 g/m(2)day as an annual average, leading to high suspended solids (SS) contents, due to the large lake area and its shallowness (high 'Dynamic Ratio'). Sediment resuspension behaviour was further investigated in preliminary laboratory experiments using a 'micro-flume', applying increasing water currents onto five Lake Markermeer sediments. Resuspension showed a clear exponential behaviour. Finally, a 3-D model was set up for water quality and SS contents in Lake Markermeer; first results showed a good agreement between modelled and actual SS contents. Construction of artificial islands and dams will reduce wind fetches and may be expected to cause a substantial decrease in lake water turbidity.     

Sedimentation dynamics within a large shallow lake and its role in sediment transport in a continental-scale watershed

A comprehensive understanding of the sedimentation dynamics within Lake Winnipeg (surface area: 23,750 km²) and its role in sediment transport in the downstream river system was achieved by determining the properties of lake bottom sediment and patterns of sediment accumulation rates and by constructing a conceptual total (i.e., organic and inorganic) sediment budget. Net deposition was the governing process in the South and North Basins, whereas transportation dominated in the Narrows. The largest fluvial source of sediments to the lake, the Red River, supplies 35% of the total sediment load. Although accumulation rates in profundal zones progressively decreased northward from this source at the south end of the lake, high accumulation rates with low inventories of fallout radionuclides in the northern margin of the North Basin indicate a second sediment source, which was determined to be erosion of north shore banks, which accounts for up to 50% of the total sediment load to the lake. The nearshore-offshore gradient in bottom sediment properties in the North Basin confirmed that the signature of this source can reach at least 20 km southward into the lake. However, the properties of bottom sediments, sedimentation dynamics, and sediment budget suggested that some of the materials eroded from the north shore are exported without interaction with the lake bottom and this local sediment source is the dominant source for the downstream river system. It was concluded that Lake Winnipeg effectively disconnects the downstream Nelson River from sediment transport processes in its upstream watershed (953,250 km²).     

Sediment Resuspension and Currents in Lake Manitoba

The physical resuspension of bottom sediment during periods of high winds is thought to be important for nutrient recycling in large shallow lakes. In order to study resuspension, prototype instrumentation designed to collect gram quantities of suspended sediment at 38.1-cm intervals in the water column was deployed in 4.2 m of water in the shallow southern basin of Lake Manitoba for 1-month periods in 1978 and 1981. At about 2.7 m depth a sharp discontinuity was observed in the vertical distribution of total mass of suspended sediment collected. The mean current speed varied linearly with depth above the discontinuity but was more uniform near bottom. Although the water column was not thermally stratified, drogues frequently displayed a two-layer structure in the currents with a high skew in direction between layers. The particle size distributions were similarly discontinuous in the vertical, with large quantities of sand in the upper layer samples and a high silt load in the lower. The unusual particle size distribution, which was explained in terms of the origin of resuspended sediments and subsequent transport by lake currents, suggests a decoupling of lake turbulence between the two layers during high winds.     

Escherichia coli survival and transfer in estuarine bed sediments

Bed sediment resuspension is a potential source of faecal microorganisms in the water column of estuaries. As such, it is important to identify the survival of faecal microorganisms in these bed sediments and understand how bed sediment resuspension impacts the quality of estuarine waters. This study explores the effect of bed sediment resuspension on Escherichia coli concentrations in the water column and the persistence of E. coli in the water column and bed sediments of the Yarra River estuary in South‐Eastern Australia. Using sediment cores, we identified that the resuspension of both surficial sediments (e.g., by tidal movements) and deeper bed sediments (e.g., by large storm events) can increase E. coli concentrations in the water column by up to 20 times in estuaries in oceanic climates. Bed sediment resuspension can result in increased E. coli concentrations in the water column even up to 24 days after E. coli first enters the estuarine water. This study demonstrates that faecal microorganisms, such as E. coli, can persist for extended periods in estuarine bed sediments, which may then be re‐entrained into the water column via recreational activities, high flow events, or tidal fluctuations. If the survival and resuspension processes observed here hold true for pathogenic microorganisms, the resuspension of bed sediments may indeed represent an increased public health risk.     

Satellite remote sensing reveals impacts from dam‐associated hydrological changes on chlorophyll‐a in the world's largest desert lake

We present an approach that uses satellite products to derive models for predicting lake chlorophyll from environmental variables, and for investigating impacts of changing environmental flows. Lake Turkana, Kenya, is the world's largest desert lake, and environmental flows from the Omo River have been modified since 2015 by the Gibe III dam in Ethiopia. Using satellite remote sensing, we have evaluated the influence of these altered hydrological patterns on large‐scale lake phytoplankton concentrations for the first time. Prior to dam completion, strong seasonal cycles and large spatial gradients in chlorophyll have been observed, related to natural fluctuations in the Omo River's seasonal discharge. During this period, mean lake chlorophyll showed a strong relationship with both river inflows and lake levels. Empirical models were derived which considered multiple hydro‐climatic drivers, but the best model for predicting chlorophyll‐a was a simple model based on Omo River discharge. Application of this model to data for 2015–2016 estimated that during the filling of Gibe III annual mean Lake Turkana chlorophyll declined by 30%. Future water management scenarios based on Gibe III operations predict reduced seasonal chlorophyll‐a variability, while irrigation scenarios showed marked declines in chlorophyll‐a depending on the level of abstraction. These changes demonstrate how infrastructure developments such as dams can significantly alter lake primary production. Our remote sensing approach is easy to adapt to other lakes to understand how their phytoplankton dynamics may be affected by water management scenarios.     

Storm-induced Redistribution of Deepwater Sediments in Lake Ontario

High-resolution seismic reflection profiles, side-scan sonar profiles, and surface sediment analyses for grain size (% and, silt & clay), total organic carbon content, and carbonate content along shore-perpendicular transects offshore of Olcott and Rochester in Lake Ontario were utilized to investigate cm-thick sands or absence of deepwater postglacial sediments in water depths of 130 to 165 m. These deepwater sands were observed as each transect approached and occupied the “sills,” identified by earlier researchers, between the three deepest basins of the lake. The results reveal thin (0 to 5-cm) postglacial sediments, lake floor lineations, and sand-rich, organic, and carbonate poor sediments at the deepwater sites (> 130 m) along both transects at depths significantly below wave base, epilimnetic currents, and internal wave activity. These sediments are anomalous compared to shallower sediments observed in this study and deeper sediments reported by earlier research, and are interpreted to indicate winnowing and resuspension of the postglacial muds. We hypothesize that the mid-lake confluence of the two-gyre surface current system set up by strong storm events extends down to the lake floor when the lake is isothermal, and resuspends and winnows lake floor sediment at these locations. Furthermore, we believe that sedimentation is more likely to be influenced by bottom currents at these at these sites than in the deeper basins because these sites are located on bathymetric highs between deeper depositional basins of the lake, and the bathymetric constriction may intensify any bottom current activity at these sites.     

Stimulation of Lake Michigan Plankton Metabolism by Sediment Resuspension and River Runoff

Previous work during a major sediment resuspension event (March 1988) in southern Lake Michigan demonstrated that nutrients and carbon derived from resuspended sediment stimulated intense winter heterotrophic production while simultaneously decreasing light availability and autotrophic biomass. However, the role of riverine inputs on plankton metabolism remained unclear. Here we present results from a simulated enrichment experiment (March 2000) designed to examine the influence of resuspended sediments and riverine inputs on Lake Michigan plankton dynamics. Lake water amended with realistic levels of river water, coastal resuspended sediment and river water + sediment all showed enhanced heterotrophic bacterial production and plankton respiration rates, relative to the lake water control. Bacterial production increased by approximately 4× in river water treatments and by a factor of 2.5× for the sediment only treatment compared to lake water controls. Rates of net primary production were stimulated by river water (8.5×) and resuspended sediment (3×), but most by a combination of river water + sediments (11×). Community respiration showed a similar response with rates approximately 8x higher in river water amendment treatments and 3.5× higher in the sediment treatment. Extrapolating experimentally determined production rates to the southern Lake Michigan basin indicated that heterotrophic and autotrophic production in this nearshore region may be enhanced by as much as 3× and 5.2× due to these source inputs. Indeed, field measurements throughout southern Lake Michigan from 1998–2000 support these experimental results. Experimental and field observations suggest that both seasonal riverine inputs and episodic resuspended sediments influence the regional scale ecosystem metabolism and biogeochemistry in Lake Michigan.     

Modelling sediment accumulation and dispersion of contaminants in Lake Ijsselmeer (The Netherlands)

In this study two models are integrated for the simulation of sediment distribution and quality in the Lake Ijsselmeer area. STRESS-2d, a two dimensional dynamic model for simulation of sediment transport due to resuspension, erosion, sedimentation and horizontal advection and dispersion is used to simulate the sediment transport dynamics for a period of one year. The model is calibrated on water level, suspended solids concentration and sedimentation flux measurements. The model has a high spatial and temporal resolution. The model DIASPORA, which is based on STRESS-2d results, simulates the effects of sediment transport on morphology and contaminant concentrations in the sediment for a period of decades. DIASPORA also simulates dilution by internally produced CaCO₃ and consolidation of sediment layers. The models produce a reasonable reconstruction of suspended solids concentrations and long term accumulation of sediments in deep areas within the lake. Also the temporal and spatial variability in the contaminant concentration in sediments in Lake Ijsselmeer is reconstructed with sufficient quality. Model simulations show that internal redistribution of old deposits in the Usselmeer area and internal production of CaCO3 are diluting the contaminated solids supplied by the river Ijssel.     

Sediment TMDL Development for the Amite River

The Amite River is recognized as one of the 15 water bodies impaired by sediments in Louisiana, USA. Based on US EPA’s Protocol sediment TMDL (Total Maximum Daily Load) development is conducted for the Amite River and described in this paper. The TMDL development consists of four components: (1) development of a new model for cohesive sediment transport, (2) estimation of sediment loads (sources) due to watershed erosion, (3) river flow computation, and (4) determination of sediment TMDL for the Amite River. Using the mass conservation principle and Reynolds transport theorem a new 1-D model has been developed for computation of suspended cohesive sediment transport. Sediment erosion in the Amite River Basin is calculated by combining the USLE (Universal Soil Loss Equation) model with GIS and the digital elevation model of the Amite River Basin. Digital elevation data was imported into the GIS which generated inputs for USLE. The calculated average annual rate of soil erosion in the Amite River Basin is 13.368 tons per ha, producing a nonpoint sediment load of 103 mg/L to the Amite River. The flow computation is performed using the HEC-RAS software. The computed sediment concentration in the Amite River varies in the range of 3–114 mg/L and sediment TMDL is 281.219 tons/day. The reduction necessary to support beneficial uses of the river is 55% or 275.946 tons/day. Results indicate that the combined application of the new 1-D sediment transport model, GIS, USLE model, and HEC-RAS is an efficient and effective approach to sediment TMDL development.     

Wave Action and Bottom Shear Stresses in Lake Erie

For Lake Erie, the amplitudes and periods of wind-driven, surface gravity waves were calculated by means of the SMB hindcasting method. Bottom orbital velocities and bottom shear stresses were then calculated using linear wave theory and Kajiura's (1968) turbulent oscillating boundary layer analysis. These calculations were made for south-west and north wind directions and a steady wind speed of 40 km hr^?1. Calculated bottom shear stresses are related to the textural properties of surficial Lake Erie sediments as determined by Thomas et al. (1976). The bottom shear stresses, especially under prevailing southwest wind conditions, control the textural characteristics of surface sediments in Lake Erie. In particular, wave-induced bottom shear stresses are probably the most important energy source for sediment entrainment.     

Redox-related release of phosphorus from sediments in large and shallow Lake Peipsi: Evidence from sediment studies and long-term monitoring data

In large and shallow lakes, the role of the redox-related release of phosphorus (P) from sediments has remained in the shadow of sediment resuspension. In the current study, we concentrated on this knowledge gap regarding factors controlling lake water quality. We combined long-term monitoring data with the studies on sediment P mobility in August 2018 by measuring redox potential, pore water concentrations of soluble reactive phosphorus (SRP), dissolved iron (Fe), sediment P fractions, and by calculating diffusive P flux. Using lake water total P (TP) concentrations for 21 years (1997–2018), we quantified internal P load based on water column summer increase of TP (IL_{in situ}). Significant positive correlations were found between the diffusive P flux and the Fe-bound P concentration in the sediment for conditions of well-oxidized sediment surfaces. The analysis of long-term data showed that P mobilized in sediments is likely to be released via sediment disturbances. Sediment resuspension is favoured by decreased water level during late summer-early autumn. Additionally, the release of P from anoxic sediment surfaces is also possible, as was indicated by significant positive correlations of IL_{in situ} with the anoxic factor (a measure of extent of anoxia) and August water temperature. The potential P release from anoxic sediment surfaces contributed about 80% to IL_{in situ} in the northern basin, and about 280% in the more productive southern basin. Hence, the redox-related P release seems to sustain the high productivity of these large and shallow lake basins and is supported by sediment resuspension as a transport mechanism.     

What role does stratification play during winter in wind-induced exchange between the multi-depth basins of a large lake (Lake Geneva)?

Detailed studies of wind-driven interbasin exchange in lakes, focusing on the underlying driving forces and how they are affected by stratification, are presently lacking. We therefore investigated how stratification modifies wind-induced exchange between the Petit Lac (PL) (depth 75 m) and Grand Lac (GL) (depth 309 m) basins of Lake Geneva in winter, using field observations, 3D hydrodynamic modeling and particle tracking. Early, weakly-stratified (December) and late, fully-mixed (March) winter conditions in the PL were compared for a typical, strong along-axis wind forcing. During early winter, two-layer exchange develops between the basins, with downwind surface outflow into the GL balanced by intense bottom inflow of deep, cold hypolimnetic GL water into the PL which is enhanced by baroclinic pressure gradients caused by upwelling in the GL. Furthermore, the transversal water-level setup generates barotropic pressure gradients that balance Coriolis force acting on the outflow. This produces unidirectional along-wind epilimnion currents that strengthen interbasin exchange. In late winter, with the thermocline deeper than the PL bottom, upwelling in the GL does not reach the confluence and baroclinicity plays no role, resulting in weaker exchange currents with a depth-veering structure in the upper layers due to Coriolis force. In late winter, interbasin exchange decreases by 50 %, is more local, affects only waters near the confluence, and is more horizontal, with no deepwater upwelling from the GL. Our results suggest that prolonged winter stratification due to global warming will make wind-induced hypolimnetic interbasin-upwelling an increasingly important deepwater renewal process in large multi-depth basin lakes.     

Near-Bottom Currents and Suspended Sediment Concentration in Southeastern Lake Michigan

In a study of sediment transport at the edge of the coastal shelf (28 m depth) in southeastern Lake Michigan we used an instrumented tripod to make continuous observations of horizontal current velocity, temperature, and turbidity within 1 meter of the bottom for 4 weeks during October 1981. The concentration of total suspended material (TSM) 0.9 m above the bottom varied from 1 to 5 mg/L in response to coastal upwelling, surface waves, and currents that exceeded 0.28 m/s (0.7 m above the bottom) on occasion. Advection of the Grand River plume also contributed significantly to the variations in the observed TSM concentration. Currents near the bottom were well correlated with surface winds and, although upwelling currents transported sediment upslope, the net horizontal sediment flux during the period of observation was west-southwestward, almost directly offshore. The magnitude of the horizontal sediment flux was approximately 1,000 times the magnitude of the vertical flux estimated from sediment traps deployed as part of earlier studies. We infer that local resuspension occurred roughly 20 percent of the time and the critical mean flow speed (at 0.7 m) for resuspension of the local silty sands was estimated to be about 0.18 m/s.     

Sediment Trap Studies in Lake Superior: Insights into Resuspension,Cross-margin Transport,and Carbon Cycling

As part of the Keweenaw Interdisciplinary Transport Experiment in Lake Superior (KITES) project, sediment traps were deployed at multiple locations along the northern coast of the Keweenaw Peninsula in Lake Superior. Traps were deployed at multiple depths (25 m below water surface to 5 m above sediments) at varying distances from shore (0.5–21 km) during the months May-October over a 3-year period. Material captured in the sediment traps was analyzed for total carbon (C), total nitrogen (N), total phosphorus (P), total copper (Cu), ²¹⁰Pb, and stable isotope ratios of C and N. Copper in sediment trap material, a tracer for mine residues, indicated transport of material in both directions along the peninsula. Cross-margin transport of material occurred at all trap locations along the coast and in all seasons. The most significant finding was that sediment traps suspended just below the thermocline collected large amounts of resuspended sediments even when the trap was moored in 120–220 m of water (9–21 km from shore). Estimates of the contribution to settling fluxes of organic carbon from resuspended sediments ranged from 10–30% in offshore traps; estimates based on ²¹⁰Pb agreed well with estimates based on a carbon mixing model. Element ratios suggest that resuspended material in offshore traps originated in near-shore regions. Despite the strong influence of resuspension on the composition of sedimenting material, stable isotope ratios were controlled by processes occurring in the water column. Seasonal variations in isotope ratios may reflect seasonal shifts between predominance by autotrophic and heterotrophic processes in the water column.     

Climatology of Large Sediment Resuspension Events in Southern Lake Michigan

Lake Michigan, particularly the southern basin, is subject to recurrent episodes of massive sediment resuspension by storm-induced waves and currents. The purpose of this paper is to investigate the climatology of these events for Lake Michigan, including an analysis of associated meteorological conditions. This paper begins by examining turbidity records from two water treatment plants (Chicago, IL and St. Joseph, MI) for which long-term records are available. The turbidity records from the two plants show significant differences indicating that turbidity measurements from a single location would probably not be representative of a basin-wide climatology. A one-dimensional sediment resuspension and deposition model for fine-grained sediments is then developed and calibrated with data from the water treatment plants. The one-dimensional model is applied at 15 points around the southern basin for a 45-year period for which Lake Michigan wave climatology is available and the results are averaged to obtain a basin-wide turbidity index (Southern Lake Michigan Turbidity Index, SLMTI). A frequency distribution of the turbidity index is presented and meteorological conditions associated with the largest events are examined. Our analysis indicates that significant resuspension events in southern Lake Michigan are usually caused by a strong cyclone passing to the east of the lake. The most likely time of the year for this to occur is October to April. There is an average of 1 event per year with SLMTI above 25 mg/L and each event typically lasts about 3 days. Our analysis indicates that events have occurred more frequently since the late 1980s as the number of winter storms has increased and ice cover has decreased.