Summer Phytoplankton Community Research Articles

The effects of recent environmental changes in Lake Waahi on two forms of the common smelt Retropinna retropinna, and other biota

Since 1977 open cast (open pit) coal mining has increased discharge of silt-laden wastes into Lake Waahi, a small shallow lake in the Waikato River system, New Zealand. Phenolic wastes from a coal carbonization factory on the major tributary also have entered the lake in high but variable concentrations since 1974. During and after the summer of 1978–1979 total suspended solids and turbidity were high, the formerly dense macrophyte beds disappeared, the summer phytoplankton community changed from dominance by blue-green algae to diatoms, the eel fishery failed and the black swan population decreased to low levels. The once abundant lake resident form of the common smelt declined drastically between 1975 and 1981 and apparently was extinct by 1983. The migratory form, although abundant until 1981, was scarce by 1983. Low water levels in the outlet stream and the malfunction of a fish pass in the flood control structure located near its mouth may have impeded upstream migration in the spring of 1982. The major cause for the changes in Lake Waahi was the addition of silt; however, the loss of macrophytes may have caused increased toxicity of phenols, particularly to eels, and also promoted continual resuspension of sediments by more effective wind-produced circulation. Remedial measures are considered.

Summer phytoplankton community structure and growth in a regional upwelling area off hachijo Island, Japan

Phytoplankton community structure both in terms of taxonomic composition and size distribution, and growth were examined in a regional upwelling which occurred in a Kuroshio region in summer. Organic carbon abundance of diatoms was markedly enhanced by the upwelling and they accounted for 87% of the total phytoplankton carbon. Nitzschia pungens Grunow was numerically most prominent accompanied by Chaetoceros and Rhizosolenia species. On the other hand, only a small increase was observed in phytoplankton other than diatoms, which consisted of unicellular cyanobacteria, coccolithophorids, gymnodinoids, and so called monads and flagellates. Division rate of diatoms from the upwelled water was twice that of the other forms. This difference in growth response among phytoplankton groups resulted in a shift of community structure to dominance of diatoms from that of the other forms. The time needed for this shift was estimated to be at least ≈ 3 days.

Accelerated eutrophication in a New Zealand lake: Lake Rotoiti, central North Island

Abstract Lake Rotoiti, North Island, New Zealand, is a deep (maximum depth, 126 m) lake surrounded by a mostly forested and farmland catchment. However, it currently demonstrates several attributes characteristic of enriched waters. Algal production is high relative to other deep basin lakes of the central North Island. The hypolimnion deoxygenates completely each year, and a bloom‐forming species of blue‐green algae is numerically important in the summer phytoplankton. These features appear to be recent rather than longstanding properties of the Lake Rotoiti ecosystem. A detailed comparison of limnologieal measurements from 1955 onwards reveals a consistent decline in water clarity, a large and possibly exponential increase in hypolimnetic oxygen demand, and a conspicuous shift in the summer phytoplankton community towards blue‐green algal dominance. This recently accelerated change in the trophic status of Lake Rotoiti is probably a response to the Ohau Channel inflow from Lake Rotorua.

Relationships between phytoplankton periodicity and the concentrations of total and unionized ammonia in channel catfish ponds

Phytoplankton abundance fluctuated throughout a 9 month study period in two 1.62 ha channel catfish (Ictalurus punctatus) production ponds. Summer phytoplankton communities in both ponds were dominated by blue-green algae (Cyanophyta). During periods of rapidly decreasing phytoplankton abundance, total ammonia-nitrogen concentrations increased but a decrease in pH during these same periods moderated the concentrations of unionized ammonia-nitrogen. Although unionized ammonia-nitrogen concentrations did not reach levels considered acutely toxic to fish, extended periods of moderately high unionized ammonia-nitrogen concentrations (>O.15 mg/1) occurred in the late fall months and may have serious implications for fish health.