Gideon Gal

Zooplankton patch dynamics: daily gap formation over abrupt topography

Abstract Net tow and acoustic surveys of zooplankton distributions were made over and around Sixtymile Bank (110 km southwest of San Diego, California). Gaps devoid of vertically migrating zooplankton were formed every evening above the summit of the bank. Interactions between the migrating animals, their predators, physical advection and the local topography appear to determine the gap formation and dynamics. Gaps were transported downstream during the night and appeared to disintegrate slowly through vertical swimming behavior, current shear and mixing processes. Patch dynamics following gap formation, mediated by both ocean currents and animal behavior, should augment the spatial heterogeneity of zooplankton and affect marine food webs in areas where abrupt topography features are common.

Hydroacoustic measures of Mysis relicta abundance and distribution in Lake Ontario

Mysis relicta can be observed on echograms as a sound scattering layer when they migrate into the water column at night to feed on zooplankton. However, quantitative measures of mysid abundance with hydroacoustics requires knowledge of mysid target strength (TS), a method of removing fish echoes and contribution from noise, and an understanding of the effect of range on the ability of hydroacoustics to detect mysids (the detection limit). Comparisons of paired net data and acoustics data from July 7, 2005 yielded a mysid TS of −86.3 dB (9 mm animal) and a biomass TS of −58.4 dB (g dry wt)−1. With ambient noise levels (S v of −125 dB at 1 m depth) and this TS, we can detect a mysid density of 1 m−3 at 60 m depth with a signal to noise ratio of 3 dB. We present a method to remove backscattering from both noise and fish and apply this method and the new TS data to whole lake acoustic data from Lake Ontario collected in July 25–31, 2005 with a 120 kHz echosounder as part of the annual standard fish survey in that lake. Mysis abundance was strongly depth dependent, with highest densities in areas with bottom depth > 100 m, and few mysids in areas with bottom depth < 50 m. With the data stratified in five bottom depth strata (> 100 m, 100-75 m, 75–50 m, 50–30 m, < 30 m), the whole-lake average mysid density was 118 m−2 (CV 21%) and the whole-lake average mysid biomass was 0.19 g dry wt m−2 (CV 22%) in July 2005. The CVs of these densities also account for uncertainty in the TS estimates. This is comparable to whole-lake density estimates using vertical net tows in November, 2005 (93 m−2, CV 16%).

Effect of land-use change scenarios on nutrients and TSS loads

The projected growth in population in Israel by 50xa0% by 2030 will greatly enhance urban density and motivate increased urbanization of rural regions in the country. The Lake Kinneret watershed is a rural region of which only about 3xa0% of the total area is used for residence, and currently, it is the least populated region in Israel. A significant land-use change and growth of urbanized regions is therefore expected in the near future, leading to changes in water quality management in the watershed. In this study, we attempted to quantify the effects of these possible changes in land-use on the flow and pollutant loads discharged from the watershed into Lake Kinneret. To that end, we calibrated and verified the AVGWLF (ArcView (GIS) Generalized Watershed Loading Function) model to simulate stream flows, sediment and nutrient loads under the conditions of a Mediterranean climate watershed. In addition to AVGWLF model, we used two external tools, namely: the HYdrological Model for Karst Environment (HYMKE) to predict daily flows of streams which were not simulated by the AVGWLF model, and a Mediterranean Multiplication Factor (MMF) which was used to improve sediment transport and nutrient load simulations. The combined suite of tools successfully simulated the observed data (r2u2009>u20090.70 and Nash-Sutcliffe efficiency >0.69 for flowrate, sediment and nutrient), including extreme values. The successful combination of the models provides watershed and lake managers the ability to examine the potential land-use changes and their impact on the watershed and the lake downstream.

Seston and Organic Matter

Studies carried out during 2000–2010 allowed us to estimate the temporal and spatial dynamics of structural and functional characteristics of seston and organic matter in Lake Kinneret: seston mass (total suspended solids, TSS), and particulate, dissolved, and total organic carbon (POC, DOC, and TOC = POC + DOC). Similar characteristics were determined for the Jordan River. On average, TSS, POC, DOC/POC, and TOC declined with water depth, indicating relative increase of seston decomposition. Phytoplankton was the largest component of the living part of seston in Lake Kinneret, comprising on average 24.7 % of the seston dry weight. During intense dinoflagellate blooms, phytoplankton comprised up to 90 % of the seston mass. Despite significant variability of the living compartments and drastic changes in the phytoplankton structure, the nonliving component was, on average, the largest component of Lake Kinneret seston, comprising about 60 % of seston mass. Metabolic activity associated with seston dominated by specific algal taxa was also estimated. The potential ecosystem importance of transparent exopolymer particles (TEP), a previously unrecognized, major component of the seston, is described. Major fluxes within the seston cycle were estimated directly, allowing the compilation of a seston budget. Sources and sinks of seston appeared to be approximately balanced which is in good agreement with the observed long-term stability of seston dynamics.

Water Quality Assessment and Management of Lake Kinneret Water Resources: Results and Challenges

Management of a human-controlled social-ecological system should be based on a set of criteria allowing a compromise between the necessity to conserve the aquatic ecosystem in some predefined “reference” state and the necessity to provide the required ecosystem services such as water supply, a recreational site, and fishery. The task of water resource management can therefore be formulated as the optimization of an objective function (Q) of the economic activities (EA): anthropogenic activities in the lake watershed and intensity of water resources uses, water quality (WQ), and economic effectiveness of the management (costs versus benefits, CB): and assessment of the relationships between them should be a central task for establishing a scientifically based water resource management strategy. In reality, however, we demonstrate the problematic aspects of such an implementation.

Protozoa (Unicellular Zooplankton): Ciliates and Flagellates

Heterotrophic and mixotrophic protozoa have been recognized as important constituents of aquatic microbial food webs since the 1990s. Much less is known about the ecosystem roles of these organisms than about bacteria and archaea. Several pioneering studies in Lake Kinneret made in the 1980s clearly indicated that heterotrophic and mixotrophic ciliates and flagellates were of great significance in the mineralization of organic matter and cycling of carbon, nitrogen, and phosphorus. Modeling studies suggested that the protozoa, especially ciliates, appeared to be a critical food source for copepods. However, only after many years there was renewed research on protozoa as drivers of carbon flux and nutrient cycling. Routine monitoring of the lake ciliate populations was begun in 2006.