Effects of nutrient (N,P, C) enrichment,grazing and depth upon littoral periphyton of a softwater lake

Three field experiments were performed in Lake Lacawac, PA to determine the importance of potentially limiting nutrients relative to other factors (grazing, depth) in structuring shallow water algal periphyton communities. All three experiments measured periphyton growth (as chlorophyll-a, AFDM or biovolumes of the algal taxa) on artificial clay flower pot substrates which released specified nutrients to their outer surfaces.Control of standing crop by nutrient supply rate vs. grazing was examined in Expt. I. Substrates releasing excess N and P, together with one of 4 levels of C (as bicarbonate) were placed either inside or outside exclosures designed to reduce grazer densities. Chlorophyll-a rose from 1.1–25.6 µg.cm^–2, and some dominant taxa (e.g., Oedogonium, Nostoc, Anacystis) were replaced by others (e.g., Scenedesmus, Cryptomonas) as bicarbonate supply increased. Reductions in invertebrate density did not significantly affect chlorophyll-a at any of the nutrient levels.Reasons for the species shift were further evaluated in Expt. II, using a minielectrode to measure the elevation of pH within the periphyton mat through photosynthetic utilization of bicarbonate. The pH adjacent to pots diffusing N, P and large quantities of bicarbonate, and supporting high chlorophyll-a densities of 32 µg cm^–2, averaged 10.0 compared to 6.3 in the water column. Pots diffusing only N and P supported 0.7 µg chlorophyll-a cm^–2 and elevated pH to 8.2. We suspect that bicarbonate addition favored efficient bicarbonate users (e.g., Scenedesmus), while inhibiting other taxa (e.g., Oedogonium) because of the attendant high pH.Expt. III was designed to test effects of depth (0.1 m vs. 0.5 m) and N (NH₄ ⁺ vs. NO₃ ^–) upon the growth response to bicarbonate observed in Expts. I and II. Similar standing crop and species composition were noted on pots at 0.1 m vs. 0.5 m. Enrichment with NH₄ ⁺ vs. NO₃ ^– also appeared to have little effect upon the periphyton community.Shallow water periphyton communities in Lake Lacawac, when supplied with sufficient N and P, appear to show a distinctive response to increasing bicarbonate concentration and pH which is robust to moderate variation in grazer densities, distance from the water surface, and the form of N enrichment.     

Floristic altitudinal zones in Malesia

From the altitudinal ranges of species recorded for the Malesian mountain flora, in Flora of Java and the 6 or so volumes of Flora Malesiana it was concluded that there are critical altitudes where the floristic composition shows rather abrupt demarcations, namely at 1000, 1500, 2400 and 4000 m altitude. In a generalized way this also holds for the structure and stature of the vegetation. For cultivated plants the altitudes of 1000 and 1500 m are also distinct demarcations.     

Growth interactions between littoral diatoms and juvenile marine algae

Interactions have been studied between juvenile plants of green, brown, and red marine algae and 31 diatom clones isolated from a variety of marine eulittoral habitats. The interactions seemed to be of an individual nature for both juvenile plants and diatoms. Germlings of Ulva lactuca L. mostly showed enhanced growth, often with significant increases in population sizes of the accompanying diatoms. Fucus spiralis L. germlings were mainly unaffected by growth in the presence of the diatom clones, but growth of the diatoms was often stimulated. Ascophyllum nodosum (L.) Le Jol. germlings were little affected, whilst the accompanying diatoms were less noticeably affected than with Fucus spiralis. Germlings of F. vesiculosus L. often showed growth inhibitions in the presence of diatoms, with many diatom populations showing enhanced growth. Similarly, the discoid encrusting sporelings of Chondrus crispus Stackh. showed growth inhibitions where there were measurable interactions, although the accompanying diatoms usually failed to show growth stimulations. The discoid sporelings of Gigartina stellata (Stackh. in With.) Batt. showed high mortalities, usually with marked increases in population sizes of accompanying diatoms.     

The macroinvertebrate fauna of littoral and bottom substrata as a tool for biotypology of Frisian waters

During a biotypological research of surface waters in the province of Friesland, The Netherlands, the macroinvertebrate fauna was sampled, both from the littoral and bottom substrata. Results of this investigation in different types of water on 60 and 55 stations in 1981 and 1982, respectively, are presented. The species richness in the littoral samples exceeded that of the bottom samples, and the samples per station had relatively few species in common. Even in many cases the most abundant taxon was a different one in both samples of the same spot. With multivariate analysis of the data sets from bottom, littoral and total samples a fairly good resemblance was found between these data sets. It is concluded that the semi-quantitative processing of macroinvertebrate fauna data leads to reliable results. One can use one sample per station, wherein different substrata are combined. Distinct clusters were recognized in more extreme environmental conditions such as brackish and acid waters. With the help of the macroinvertebrate fauna the other types of water could not easily be differentiated.     

Effect of oxygen availability and salinity on EARLY LIFE HISTORY STAGES OF SALT MARSH PLANTS. I. Different germination strategies of Spartina alterniflora and Phragmites australis (Poaceae)

Gradients in oxygen availability and salinity are among the most important environmental parameters influencing zonation in salt marsh communities. The combined effects of oxygen and salinity on the germination of two salt marsh grasses, Spartina alterniflora and Phragmites australis, were studied in growth chamber experiments. Germination of both species was initiated by emergence of the shoot and completed by root emergence. Percentage S. alterniflora germination was reduced at high salinity (40 g NaCl/L) and in decreased oxygen (5 and 2.5%). In 0% oxygen shoots emerged, but roots did not. P. australis germination was reduced at a lower salinity (25 g NaCl/L) than S. alterniflora, and inhibited at 40 g NaCl/L and in anoxia. However, a combination of hypoxia (10 and 5% O2) and moderate salinity (5 and 10 g NaCl/L) increased P. australis germination. When bare areas in the salt marsh are colonized, the different germination responses of these two species to combinations of oxygen and salt concentrations are important in establishing their initial zonation. In high salinity wetlands S. alterniflora populates the lower marsh and P. australis occupies the high marsh at the upland boundary.     

Effect of vertebrate predation on the spatio-temporal distribution of cladocerans in a temperature eutrophic lake

We analysed the spatio-temporal distribution of zooplankton along a profile of 10 stations from the shore to the pelagic zone from April to September 1988, the period when the larvae and juveniles Rutilus rutilus, the most abundant species in the Lake, are in the littoral zone. The digestive tracts of the young roach were analysed. They fed essentially on rotifers and on cladocerans. For comparison, zooplankton was also analysed at one littoral area without fish fry. There was an increase of cladoceran density from the vegetated nearshore zone to the offshore zone. Considering the density of Bosmina longirostris, Daphnia longispina, Chydorus sphaericus and Ceriodaphnia quadrangula, we observed a different distribution pattern in the course of the year. In the nearshore zone, the relative abundance of small species, Bosmina and Chydorus, was much higher than that of the larger Daphnia. From April to September, predation pressure mainly affected the smallest species: in contrast to the inshore station without fish fry, the density of Bosmina decreased in May in the littoral with fish. Chydorus was concentrated in the littoral between February and April, then grew into the pelagic zone, where predation pressure obviously was low during the warm season. The number of Daphnia, which was eaten by the fish fry at any time, remained low in the nearshore zone, which suggests that the presence of fish may cause Daphnia to avoid this zone. Ceriodaphnia which was not affected by this predation, was scarce in the nearshore zone during mid-summer. The low density of the cladocerans in the nearshore zone is likely associated with vertebrate predation by roach fry and juveniles, the result of such a process being either a depletion in density of the prey, or an avoidance behaviour.