Trophic state of lobelia lakes subjected to strong anthropogenic pressure located close to the Tricity area (northern Poland)

Intensive and expansive human activity greatly accelerates the natural process of lake evolution. It is assumed that an increase in the rate of lake fertilization (often termed “eutrophication”) is the most common and most serious anthropogenic disturbance occurring in these aquatic geoecosystems. Lobelia lakes, one of the most valuable water bodies, are particularly exposed to this type of pressure. The article presents the trophic state of 13 lobelia lakes, located close to the Tricity area, in the northeastern part of the Kashubian Lakeland, which are not legally protected. These lakes represent different geoecosystems determining the rate of their natural evolution, and are subjected to various forms of anthropopressure (numerous summerhouses, fishing, waterside leisure and recreation). The trophic status of these lakes was assessed on the basis of the Carlson-type indices. The evaluation indicated that six of these lakes were in borderline mesotrophic-eutrophic state, and seven represented a significantly advanced eutrophic state.

Sockeye salmon (Oncorhynchus nerka) nursery lake fertilization: Review and summary of results

We reviewed 24 sockeye salmon (Oncorhynchus nerka) nursery lake experiments that involved whole-lake fertilization with appropriate treatment and control years. We found that: 21 of 21 studies showed that fertilization was associated with increased chlorophyll a concentrations, 16 of 16 showed increased zooplankton biomasses, 16 of 16 demonstrated increased average smolt weights, and 11 of 13 showed increased smolt biomasses. Studies involving assessments of egg-to-smolt survival were rare, but all (4 of 4) showed increased survival rates. Studies involving increased smolt-to-adult survival (i.e., marine survival) were even rarer, but all (3 of 3) showed that lake fertilization and increased smolt size were associated with increased marine survival. Several fertilization studies reported problems, and some offered solutions. For instance, when whole-lake fertilization stimulated the growth of blue-green algae, fertilizer with higher nitrogen to phosphorus ratios was used to control the problem. Conversely, when high nitrogen to phosphorus ratios were associated with blooms of ungrazable diatoms, notably Rhizosolenia eriensis, reduced nitrate concentrations were recommended. To date, solutions designed to constrain the growth of both blue-green algae and Rhizosolenia blooms remain elusive. Some studies showed that when both mysids (large invertebrate planktivores) and juvenile sockeye inhabit the same lake, sockeye suffer from a competitive disadvantage and mysids consume 80–90% of the available zooplanktonic food production. Similarly, a small number of studies demonstrated that competition from sticklebacks (Gasterosteus aculeatus) adversely affected sockeye growth rates, and although the problem remains unresolved, ongoing work in lakes containing kokanee (O. nerka), suggests that stocked cutthroat trout (Salmo clarki) may be capable of controlling stickleback densities through predation. Despite all of these difficulties, in almost all cases, when lakes were fertilized with various mixtures of inorganic nitrogen and phosphorus, pelagic food web bottom-up control was strong enough and predictable enough to ensure that sockeye smolt biomass increased. We conclude that sockeye nursery lake fertilization is a technique that can contribute usefully to both the enhancement and conservation of sockeye salmon populations. Key words: sockeye salmon, lake fertilization, bottom-up, aquatic food web.

Effects of epilimnetic versus metalimnetic fertilization on the phytoplankton and periphyton of a mountain lake with a deep chlorophyll maxima

Nutrients can load directly to either the epilimnion or metalimnion of lakes via either differential inflow depths of tributaries or intentional fertilization of discrete strata. We evaluated the differential effects of epilimnetic versus metalimnetic nutrient loading using 17-m-deep mesocosms that extended into the deep chlorophyll layer of oligotrophic Pettit Lake in the Sawtooth Mountains of Idaho. Addition of nitrogen plus phosphorus stimulated primary production nearly identically (2.4- to 4-fold on different dates) in both treatments, with the production peaks occurring in the strata where nutrients were added. The metalimnetic fertilization, however, resulted in equal or greater stimulation of chlorophyll a and phytoplankton biovolume than when nutrients were added directly to the epilimnion. Periphyton growth was stimulated 10–100 times more by epilimnetic fertilization than by metalimnetic fertilization and diverted nutrients from the planktonic autotrophs. These results suggest that the development of deep chlorophyll layers may be influenced by plunging river inflows that carry nutrients to the metalimnion and that metalimnetic lake fertilization may be useful as a tool for increasing lake productivity while reducing the impact on water quality.