Hunter J. Carrick

Can wind-induced resuspension of meroplankton affect phytoplankton dynamics?

Data from Lake Apopka, Florida, are presented to illustrate the importance of wind-induced resuspension of sediments in a shallow, subtropical, hypereutrophic lake. Wind-induced resuspension of meroplankton, primarily planktonic diatoms that settle to the benthic environment, accounts for a large component of the temporal variation in phytoplankton biomass. Chlorophyll concentrations >100 μg/L are highly correlated with wind speed, and the regression of wind speed on chlorophyll accounts for 53% of the temporal variability in chlorophyll. Resting cells of diatoms are also resuspended with the meroplanktonic community. Induction of resting cells that are physiologically dormant occurs in darkness; but within a few hours after exposure to the water-column light and nutrient environment, resting cells become physiologically active. Other types of resting propagules in addition to diatom resting cells also may be resuspended from sediments and affect phytoplankton dynamics. Data from Lake Apopka and other systems are used to develop paradigms about time and depth scales for resuspension of meroplankton and resting propagules in lakes and in the sea.

Photosynthetic Characteristics of Phytoplankton Communities in Lakes Huron and Michigan: P-I Parameters and End-Products☆

Photosynthetic-irradiance (P-I) curves and partitioning of photosynthate into major end-products (protein, lipids, polysaccharides, and low molecular weight [LMW] metabolites) were examined for phytoplankton communities from Lakes Huron and Michigan. The mean and variance of P-I parameters and photosynthetic end-products were similar in both lakes. Mean PMB (maximum light saturated rate) and α (initial linear slope) values were 2.3 mg C·mg Chl−1·h−1 and 5.5 mg C·mg Chl−1·Einst−1·m2 for Lake Huron communities, and 2.4 mg C·mg Chl−1·h−1 and 7.0 mg C·mg Chl−1·Einst−1·m2 for Lake Michigan communities. The mean percent incorporation of 14CO2 into proteins, lipids, polysaccharides, and LMW metabolites from short-term experiments (2-4 h) were 32.4, 21.3, 28.0, and 18.9, respectively, for Lake Huron communities and 34.8, 24.7, 24.5, and 15.8, respectively, for Lake Michigan communities. Over longer incubations the activity in each end-product increased linearly during the day; during the night the activity in the LMW and polysaccharide fractions decreased and the activity in the protein fraction increased. There were significant seasonal variation in P-I parameters and the photosynthetic end-products. In both lakes, phytoplankton communities from the late winter-spring isothermal period were characterized by lower PMB values, higher α values, significant susceptibility to photoinhibition, and less incorporation into protein, as compared to communities from periods when the lakes were thermally stratified.

Guilds of Benthic Algae along Nutrient Gradients: Relationships to Algal Community Diversity

We attempt to define groups of functionally related benthic algal species or guilds to assess if the species richness of such guilds varies across experimentally manipulated nutrient gradients, and to determine the relative contribution of these guilds to total community diversity. Nutrient gradients were established using nutrient-releasing substrata; treatments consisted of Si, N+P, Si+N+P, and controls. Nutrient enrichment significantly altered the biovolume of 27 species (out of a total of 141). Results from one-way ANOVA tests coupled with multiple means range tests categorized these species into four major guilds: three guilds of species which achieved their highest abundance on either Si, N+P, or Si+N+P treatments, and a guild that grew best on controls. This pattern of structuring was corroborated by cluster analysis and principal components analysis. Total community diversity and the relative contribution of guilds to total community diversity was less on N+P and Si+N+P substrata compared with that on Si and the control substrata. This suggests that nutrient enrichment may narrow the conditions amenable to many algal species (nutrient generalists), creating a niche occupied by those taxa sufficiently equipped to benefit under such conditions (nutrient specialists).

Planktonic Protozoa in Lakes Huron and Michigan: Seasonal Abundance and Composition of Ciliates and Dinoflagellates

The abundance and biomass of surface (5 m) and deep (30–45 m) ciliate and dinoflagellate protozoa in the offshore waters of Lakes Huron and Michigan were determined from December 1986 to November 1987. Protozoan (ciliates and dinoflagellates) abundance (4 to 15 cells·mL−1) and biomass (13–140 μg·L−1, wet wt.) were comparable between lakes and similar to those reported from other oligotrophic environments. On average, ciliates comprised the majority of protozoan abundance (80%) and biomass (73%). The mean size (ESD) of these communities was small (20.6 μm) due to the numerical dominance of small choreotrichs, oligotrichs, and species of Gymnodinium. Total biomass in both lakes peaked during late June-July and again during the October-November period. These seasonal changes in biomass were accompanied by species replacements: tintinnids, strobilids, oligotrichs, and Gymnodinium species were abundant in the spring isothermal period, shifting to oligotrich dominance during summer stratification (May–July); a more diverse assemblage followed during late stratification (October–November) in which haptorids, prorodontids, and Peridinium species became more important. Deep and surface communities were comparable in terms of abundance and biomass, although deep community biomass decreased as stratification intensified. Because the biomass of ciliates alone represents approximately 30% of crustacean zooplankton biomass, protozoa may be more important grazers than once thought.

Phytoplankton community photosynthesis and primary production in a hypereutrophic lake, Lake Apopka, Florida

A study of in situ primary productivity (light- and dark-bottle oxygen exper- iments) was conducted biweekly in 1990 and 1991 to measure volumetric rates of phytoplankton photosynthesis and estimate integral phytoplankton photosynthesis and primary production. Concurrently, ambient conditions including water temperature, photosynthetically active radiation (PAR), Secchi disc transparency, dissolved oxygen and phytoplankton biomass (chlorophyll- a) were measured. Results of both light- and dark-bottle experiments are used to 1) demonstrate that Lake Apopka is not hetero- trophic as reported in the literature, 2) provide preliminary estimates of phytoplankton primary production, 3) discuss shortcomings in experimental methodology and 4) evaluate sources of temporal variability in phytoplankton primary production. No strong seasonal patterns were found in ambient physical and chemical conditions with the exception of water temperature. Whole-basin estimates of net organic carbon sedi- mentation are used as an independent approach to estimate phytoplankton primary pro- duction. The magnitude of net primary production ranged from approximately 1 to 2gCm -2 d -1 for three direct estimates and from to 3 to 5gC m -2 d -1 for the indirect method. We conclude that short-term temporal variability in primary productivity is controlled mainly by meteorological forcing of wind-induced turbulence and resuspen- sion of meroplankton. Our study shows that obtaining precise and accurate direct esti- mates of phytoplankton photosynthesis and production are more complex than might be anticipated from the application of standard techniques.

Importance of the microbial food web in large lakes (USA)

The traditional view of food web structure categorizes all organisms within an ecosystem into one of several feeding guilds (i.e. primary producers, decomposers, herbivores, and consumers), where energy is transferred from one guild to the next (LINDFMAN I 942). The biomass of these guilds decreases geometrically with successive trophic levels ro form a pyramid, with a large biomass of plants at the base (EI TON 1927). Metabolic inefficiencies, such as excretion and sloppy feeding, produce significant losses ar each trophic level, with the greatest loss at rhe highest trophic level (RICH & WETZFL 1978). Deviations from this paradigm have been described for planktonic communities in the ocean, where the biomass of heterorrophic organisms can rival phytoplankton, rhus indicating a tight coupling between plants and animals (e.g. OouM 1971). However, our view of food web structure in aquatic ecosystems is being further revised based upon the knowledge that small heterotrophic organisms are more quantitatively important than previously thought (AZAM et al. 1983). Recent technological advances now allow more accurate censusing of natural microbial populations, as well as measurement of their high rates of metabolic activity (KEMP ct al. 1993). Furthermore, large concentrations of non-living pools of organic carbon and detritus are common features of aquatic ecosystems, and appear to be responsible for fueling a portion of this high microbial metabolic activity (e.g. DucKLOW 1994). In this way, organic wastes produced at all levels of the food chain arc available to support significant levels of heterotrophic production, that can often rival rates of primary production (e.g. SCAVIA 1988). Despite this, comprehensive measures of heterotrophic plankton are scarce, thus making it difficult to draw inferences about their relative contribution to planktonic food web structure. Herein we present information about the planktonic food web structure in four large lakes in the United Stares. Our objectives are: (I) to estimate the biomass of heterotrophic pico-, nano-, and microplankton among lakes using rigorous microscopic enumeration techniques; (2) tO measure the variation in microheterotrophic biomass through rime (season) and space (depth); and (3) to put forth a hypothesis concerning the role of microheterotrophs in the plankton of large lake ecosystems. Our findings indicate that the biomass of microheterotrophic plankron was large in all the lakes we studied, with Hpico (bacteria) representing the largest fraction of heterotrophic biomass and exhibiting the least variance in rime and space.

Net production and net heterotrophy in Lake Apopka : a comment on SCHELSKE et al. (2003). Commentary and Authors' reply

We found the conclusion of SCHELSKE et al. (2003, Arch. Hydrobiol. 157:145-172) that Lake Apopka was not net heterotrophic was incorrect when annual rates of production and respiration for the entire lake ecosystem are taken into consideration. A new carbon budget for the water and sediments showed for the period of study the sum of the annual sources of organic carbon including primary production and inflows (765 g C m -2 yr -1 ) was less than the sum of the annual losses including respiration in the water and sediments and outflows (2497 g C m -2 yr -1 ). Diel oxygen curves demonstrated that it is possible to have oxygen supersaturation at the surface near midday and still have net heterotrophy over 24-h.