The long-term (22 yr) record of primary production in Lake Kinneret, Israel, has been examined, together with chlorophyll and microscopically determined algal biomass, in order to discern whether there have been any significant changes in these parameters during this period. During the period 1972 through 1993, annual averages have ranged from 1,223 to 2,3 11 mg C m-2 d-l for primary production, from 127 to 246 mg m-2 for chlorophyll, and from 39.3 to 98.5 g m 2 for algal wet weight. The annual peak of these parameters was in April-May. Over 22 yr, variability in primary productivity was more closely related to changes in chlorophyll than to changes in algal biomass. No evidence was found for consistently increasing long-term trends in primary production, chlorophyll concentrations, or algal wet weight biomass from 1972-1993. Although the annual and semiannual averages of algal biomass were significantly higher in the past 11 yr than those in the previous decade, this pattern could arise from a long-term cyclical but self-compensating trend. The extended record indicates that despite population growth and intense economic development around the lake and in its catchment area, there has been no extreme eutrophication of Lake Kinneret from 1972 to 1993. We suggest that this relative resiliency of the Kinneret ecosystem is due to high ambient levels of alkalinity, calcium, and pH in the lake water acting to limit phosphorus availability, which in turn restricts the outgrowth of phytoplankton.

Historically, the phytoplankton community of Lake Kinneret, Israel, has been dominated by the dinoflagellate Peridinium gatunense and other edible species that are important in the lake's food web. However, major changes have occurred both in external nutrient loading and in the water column chemistry of the lake since the mid-1980's. Epilimnetic particulate nitrogen: particulate phosphorous (PN:PP) ratios have declined, and measurements of seston chemistry suggest that the intensity of seasonal nitrogen limitation has increased. The phytoplankton community also was altered in 1994 and 1995 by a lake-wide summer invasion of the nitrogen-fixing cyanobacterium Aphanizomenon ovalisporum. This abrupt change in phytoplankton community structure is consistent with the development of conditions increasingly N-deficiency and P-sufficiency in the water column, which should favor cyanobacterial dominance.

A Global Lake Ecological Observatory Network (GLEON; www.gleon.org) has formed to provide a coordinated response to the need for scientific understanding of lake processes, utilising technological advances available from autonomous sensors. The organisation embraces a grassroots approach to engage researchers from varying disciplines, sites spanning geographic and ecological gradients, and novel sensor and cyberinfrastructure to synthesise high-frequency lake data at scales ranging from local to global. The high-frequency data provide a platform to rigorously validate processbased ecological models because model simulation time steps are better aligned with sensor measurements than with lower-frequency, manual samples. Two case studies from Trout Bog, Wisconsin, USA, and Lake Rotoehu, North Island, New Zealand, are presented to demonstrate that in the past, ecological model outputs (e.g., temperature, chlorophyll) have been relatively poorly validated based on a limited number of directly comparable measurements, both in time and space. The case studies demonstrate some of the difficulties of mapping sensor measurements directly to model state variable outputs as well as the opportunities to use deviations between sensor measurements and model simulations to better 50

The input of phosphorus (P) through atmospheric deposition can be a major source of P to fresh water bodies and may strongly affect their biogeochemistry. In Lake Kinneret (LK), northern Israel, dust deposition provides a significant fraction of the bioavailable P input. Here, we demonstrate that the oxygen isotopic composition of resin-extractable inorganic phosphate (δ(18)OP) in dust particles can be used to identify the phosphate source. Samples of soils with both natural vegetation and agricultural cover were collected upwind of LK and found to have distinct δ(18)OP value ranges (17.4-18.2‰ and 19.3-22.1‰, respectively). The δ(18)OP values for dust, collected continuously over LK during June 2011 to March 2012, were in the same range as agricultural soils. The dust concentration in the air decreased from the dry to the wet season and was correlated with a decrease in P concentration in air, yet no correlation was found between these parameters and dust δ(18)OP. Dust deposited during short-term desert dust events was characterized by a combination of high δ(18)OP values ranging from 22.2‰ to 22.7‰ and high concentrations of dust in the air. The data we present demonstrates a new application of δ(18)OP measurements for direct estimation of dust-P sources to lakes, as well as the potential for tracing dust-P on larger scales.

Physical measurements made in Lake Kinneret, during the stratified period, indicate that the lower water mass (LWM) consists of two layers: a turbulent benthic boundary layer (BBL) and a hypolimnetic layer overlying the BBL. The water in the LWM moves in response to vertical mode one seiching of the metalimnion; this movement is accentuated near the perimeter of the metalimnion due to shoaling of the seiche and breaking of internal waves. The motion associated with this phenomena induce a well mixed benthic boundary layer (BBL) adjacent to the lake bottom. The BBL and hypolimnion were distinguished chemically because transport of solutes between these water bodies was very small. Compared to the hypolimnion, the BBL was characterized by more intensive biomineralisation processes as indicated by the faster depletion of DO and NO 3 , by higher levels of NH 4 , H 2 S, and soluble reactive phosphorus (SRP) and by lower pH values. SRP levels in the hypolimnion were particularly low, suggesting that phosphorus was either preferentially removed by sedimentation from the hypolimnion, or it was not released from particles. As a result NH 4 over SRP ratios in the hypolimnion were significantly higher than in the BBL and diffusional fluxes of both nutrients from the LWM to the trophogenic epipelagic water, should therefore be characterized by relatively high N/P ratios.