Weak response of saltmarsh infauna to ecosystem-wide nutrient enrichment and fish predator reduction: A four-year study

We examined the effect of whole-ecosystem manipulations of predator removal and nutrient enrichment on saltmarsh macroinfauna in the Plum Island Estuary, Massachusetts. Nitrate and phosphate loading rates were increased 10× above background levels in experimental creeks, and we significantly reduced (by 60%) the abundance of the killifish, Fundulus heteroclitus, a key predator in this system. Two creek pairs were manipulated; Creek Pair 1 for three growing seasons and Creek Pair 2 for one. Infaunal responses were examined in four habitats along the inundation gradient: mudflat, creek wall, Spartina alterniflora, and S. patens habitats. Although benthic microalgae increased synergistically in our treatments, we detected no long-term, population-level numerical response by any taxon. Similarly, no long-term species diversity or community responses were observed. However, nutrient enrichment increased the population biomass of the polychaete Manayunkia aestuarina in the creek wall habitat and the oligochaete Cernosvitoviella immota in the S. alterniflora habitat. No numerical or biomass responses of infauna were detected in predator removal treatments although indirect effects associated with killifish reduction may have contributed to an ephemeral interaction between nutrient addition and predator reduction in S. patens habitat. Our data suggest that population and community responses between benthic microalgae and macroinfauna are not tightly coupled even though some species benefit from increased benthic algae biomass by achieving larger body size.     

Metazoan meiofauna dynamics and pelagic–benthic coupling in the Southeastern Beaufort Sea,Arctic Ocean

Pelagic–benthic coupling is relatively well studied in the marginal seas of the Arctic Ocean. Responses of meiofauna with regard to seasonal pulses of particulate organic matter are, however, rarely investigated. We examined the dynamics of metazoan meiofauna and assessed the strength of pelagic–benthic coupling in the Southeastern Beaufort Sea, during autumn 2003 and spring–summer 2004. Meiofauna abundance varied largely (range: 2.3 × 10⁵ to 5 × 10⁶ ind m⁻²), both spatially and temporally, and decreased with increasing depth (range: 24–549 m). Total meiofauna biomass exhibited similar temporal as well as spatial patterns as abundance and varied from 25 to 914 mg C m⁻². Significant relationships between sediment photopigments and various representatives of meiofauna in summer and autumn likely indicate the use of sediment phytodetritus as food source for meiofauna. A carbon-based grazing model provided estimates of potential daily ingestion rates ranging from 32 to 723 mg C m⁻². Estimated potential ingestion rates showed that meiofauna consumed from 11 to 477% of the sediment phytodetritus and that meiofauna were likely not food-restricted during spring and autumn. These results show that factors governing the distribution and abundance of metazoan meiofauna need to be better elucidated if we are to estimate the benthic carbon fluxes in marginal seas of the Arctic Ocean. This paper is dedicated to the memory of our dear friend and colleague Gaston Desrosiers who contributed so much to benthic ecology. We will continue in his spirit.     

Effects of shelter and enrichment on the ecology and nutrient cycling of microbial communities of subtidal carbonate sediments

The interactions between physical disturbances and biogeochemical cycling are fundamental to ecology. The benthic microbial community controls the major pathway of nutrient recycling in most shallow-water ecosystems. This community is strongly influenced by physical forcing and nutrient inputs. Our study tests the hypotheses that benthic microbial communities respond to shelter and enrichment with (1) increased biomass, (2) change in community composition and (3) increased uptake of inorganic nutrients from the water column. Replicate in situ plots were sheltered from physical disturbance and enriched with inorganic nutrients or left without additional nutrients. At t(0) and after 10?days, sediment-water fluxes of nutrients, O(2) and N(2) , were measured, the community was characterized with biomarkers. Autochthonous benthic microalgal (BMA) biomass increased 30% with shelter and a natural fivefold increase in nutrient concentration; biomass did not increase with greater enrichment. Diatoms remained the dominant taxon of BMA, suggesting that the sediments were not N or Si limited. Bacteria and other heterotrophic organisms increased with enrichment and shelter. Daily exchanges of inorganic nutrients between sediments and the water column did not change in response to shelter or nutrient enrichment. In these sediments, physical disturbance, perhaps in conjunction with nutrient enrichment, was the primary determinant of microbial biomass.     

Separating physical disturbance and nutrient enrichment caused by Pacific salmon in stream ecosystems

1. Pacific salmon (Oncorhynchus spp.) deliver marine‐derived nutrients to the streams in which they spawn and die, and these resource subsidies can increase the abundance of stream biota. In strong contrast, physical disturbance from salmon spawning activity can reduce the abundance of benthic organisms. Previous experimental designs have not established the relative effects of these two contrasting processes on stream organisms during a salmon run. 2. We combined manipulative and observational field studies to assess the degree of nutrient enrichment, physical disturbance, and the net effect of salmon on the abundance of benthic periphyton. Related salmon‐mediated processes were also evaluated for benthic macroinvertebrates. Mesh exclosures (2 × 2 m plots) prevented salmon from disturbing areas of the stream channel, which were compared with areas to which salmon had access. Sampling was conducted both before and during the late‐summer spawning run of pink (O. gorbushca) and chum (O. keta) salmon. 3. Streamwater nitrogen and phosphorus concentrations increased sharply with the onset of the salmon run, and highly significant positive relationships were observed between the numbers of salmon present in the stream and these dissolved nutrients. Before the salmon run, periphyton biomass (as chlorophyll a) and total macroinvertebrate abundance were very similar between control and exclosure plots. During the salmon run, exclosures departed substantially from controls, suggesting significant disturbance imparted on benthic biota. 4. Comparing exclosures before and during the salmon run enabled us to estimate the effects of salmon in the absence of direct salmon disturbance. This ‘nutrient enrichment potential’ was significant for periphyton biomass, as was a related index for macroinvertebrate abundance (although enhanced invertebrate drift into exclosures during the salmon run could also have been important). Interestingly, however, the net effect of salmon, evaluated by comparing control plots before and during the salmon run, was relatively modest for both periphyton and macroinvertebrates, suggesting that nutrient enrichment effects were largely offset by disturbance. 5. Our results illustrate the importance of isolating the specific mechanisms via which organisms affect ecosystems, and indicate that the relative magnitude of salmon nutrient enrichment and benthic disturbance determines the net effect that these ecologically important fish have on stream ecosystems.     

Bottom‐up control in the benthos of ocean‐exposed sandy beaches?

Bottom‐up control is a fundamental structuring force in food webs. Food webs of ocean‐exposed sandy beaches are predicted to be bottom‐up controlled systems, underpinned by imported organic matter rather than in situ primary production. This ecological model of resource‐based regulation of biological assemblages is juxtaposed against a prevailing paradigm built around a dominance of physical drivers in sandy beach ecosystems. Surprisingly, given the apparently ubiquitous energetic subsidies of beach food webs, the central premise of bottom‐up control has not been tested. Here we experimentally manipulated in situ nutrient levels on a sandy beach to test food web responses at the levels of primary producers (benthic microalgae) and their grazers (meiofauna). The meiofauna community as a whole appeared most strongly influenced by the local physical environment, particularly changes in sediment grain size – this supports the traditional ‘environmental control paradigm’. We also detected a significant, positive response of two consumer groups of the meiofauna (nematodes, ostracods) to nutrient enrichment that supports a model of biological, bottom‐up control. Although the predicted response of elevated producer biomass following nutrient enrichment was not detected, intense grazing pressure on new, stimulated production may have masked positive responses by the primary producers. Multichannel regulation of food webs is likely for many exposed sandy beaches, albeit an often lower importance of in situ bottom‐up forces compared with stronger environmental control.     

Above- and below-ground resource acquisition strategies determine plant species responses to nitrogen enrichment

Background and AimsKnowledge of plant resource acquisition strategies is crucial for understanding the mechanisms mediating the responses of ecosystems to external nitrogen (N) input. However, few studies have considered the joint effects of above-ground (light) and below-ground (nutrient) resource acquisition strategies in regulating plant species responses to N enrichment. Here, we quantified the effects of light and non-N nutrient acquisition capacities on species relative abundance in the case of extra N input.MethodsBased on an N-manipulation experiment in a Tibetan alpine steppe, we determined the responses of species relative abundances and light and nutrient acquisition capacities to N enrichment for two species with different resource acquisition strategies (the taller Stipa purpurea, which is colonized by arbuscular mycorrhizal fungi, and the shorter Carex stenophylloides, which has cluster roots). Structural equation models were developed to explore the relative effects of light and nutrient acquisition on species relative abundance along the N addition gradient.Key ResultsWe found that the relative abundance of taller S. purpurea increased with the improved light acquisition along the N addition gradient. In contrast, the shorter C. stenophylloides, with cluster roots, excelled in acquiring phosphorus (P) so as to elevate its leaf P concentration under N enrichment by producing large amounts of carboxylate exudates that mobilized moderately labile and recalcitrant soil P forms. The increased leaf P concentration of C. stenophylloides enhanced its light use efficiency and promoted its relative abundance even in the shade of taller competitors.ConclusionsOur findings highlight that the combined effects of above-ground (light) and below-ground (nutrient) resources rather than light alone (the prevailing perspective) determine the responses of grassland community structure to N enrichment.     

Effects of organic enrichment on meiofaunal abundance and community structure in sublittoral soft sediments

In experimental mesocosms established at Solbergstrand, Oslofjord, Norway, organic enrichment was effected by the addition of powdered Ascophyllum nodosum (L.) Le Jol., in quantities equivalent to 50gC·m^?2 and 200gC·m^?2, to boxes of sublittoral soft sediment. After 56 days, the structure of the meiofaunal communities in these treatments was compared with that of the control boxes. At this time the meiofaunal communites at each level of organic enrichment were markedly different from each other and from that in the control sediment. The responses of the two major components of the meiofauna, however, were different. Although the abundance of nematodes was slightly reduced in the high dose treatment this was not accompanied by detectable changes in community structure. Harpacticoid copepods, on the other hand, increased significantly in abundance in the treatment boxes and showed a general trend towards increased dominance and decreased diversity with increasing levels of organic enrichment, although in the low dose treatment there was also an increase in the number of species present. It is also shown that the nematode/copepod ratio is unreliable as a biomonitoring tool and it is suggested that the differential responses in community structure between the nematode and copepod components of the meiofauna might be a better indication of stress at the community level.     

Latitudinal Patterns of Herbivory in Mangrove Forests: Consequences of Nutrient Over-Enrichment

Ecosystems in the tropics are predicted to have stronger responses to nutrient enrichment, greater diversity, and more intense biotic interactions than in temperate areas. Mangrove forests, which occur across a broad biogeographic range from warm temperate to tropical, provide a unique opportunity to test these hypotheses by investigating the responses of herbivores to nutrient enrichment in temperate versus tropical latitudes. Mangroves are complex intertidal ecosystems with spatial differences in structure and diversity along tidal gradients and are threatened globally by human activities including nutrient over-enrichment. In this study, we used long-term fertilization experiments at the Indian River Lagoon, FL; Twin Cays, Belize; and Bocas del Toro, Panamá to determine how increased nutrients impact herbivore abundance and herbivory of Rhizophora mangle at the tree, forest, and regional scales. At these locations, which span approximately 2185 km and 18.4º of latitude, we fertilized individual trees with one of three treatments (Control, +N, +P) in two zones (fringe, scrub) along transects perpendicular to the shoreline and measured their responses for 4 years. Herbivory was measured as folivory, loss of yield, and tissue mining. Although nutrient enrichment altered plant growth, leaf traits, and nutrient dynamics, these variables had little effect on folivory at any location. Our results did not support the prediction that herbivory and per capita consumption are greatest at the most tropical location. Instead, folivory was highest at the most temperate location and lowest at the intermediate location. Folivory was generally higher in the fringe than in the scrub zone, but the pattern varied by location, herbivore, and nutrient treatment. Folivory by a dominant herbivore, Aratus pisonii, decreased from the highest to the lowest latitude. Our data suggest that factors controlling population dynamics of A. pisonii cascade to the mangrove canopy, linking herbivory to crab densities.     

Does bioturbation by a benthic fish modify the effects of sediment contamination on saltmarsh benthic microalgae and meiofauna?

A microcosm experiment was conducted using a replicated factorial design to determine if a benthic fish modifies the effects of sediment-bound contaminants (diesel fuel and two levels of a Cu, Cr, Pb, Hg and Cd mixture) on saltmarsh benthic primary producers and consumers. The naked goby, Gobiosoma bosc, a burrowing fish that preys on small macrofauna, was added to experimental microcosms. Goby burrowing and foraging significantly increased turbidity and disrupted the sediment surface. Results were typified by complex and varied responses with many statistically significant effects and interactions among treatments. Although G. bosc modified the responses to both diesel and metal pollution in invertebrates (but not microalgae), bioturbation did not increase or decrease the toxic effects of metals or diesel, and diesel-metal interactions did not vary in response to G. bosc. Specifically, G. bosc inhibited a trend toward diesel-induced increases in nematode abundance, and diesel toxicity inhibited increases in ostracod abundance stimulated by G. bosc. Diatoms, nematodes and the copepod Pseudostenhelia wellsi decreased in treatments with G. bosc. However, G. bosc lead to increases in cyanobacteria and ostracods and a trend toward increases in the copepod Pseudobradya sp. Our findings suggest that microcosm experiments are potentially poor mimics of natural systems without bioturbation. Conclusions about the direct and indirect effects of contaminants may differ with and without bioturbation. Finally, our work suggests that the direct effects of toxicants may inhibit or mask bioturbation effects that stimulate population growth of some meiofauna.     

Responses of algae, bacteria, Daphnia and natural parasite fauna of Daphnia to nutrient enrichment in mesocosms

Understanding responses of parasites to changes in nutrient regimes is necessary for prediction of their role in aquatic ecosystems under global change in nutrient loading. We studied the response of the natural parasite fauna of Daphnia longispina to nutrient enrichment in mesocosms in a small humic lake. We measured the concentrations of inorganic phosphorus and nitrogen in the water, total nutrients in the seston, algal and bacterial biomass, Daphnia population dynamics, Daphnia stoichiometry, Daphnia stable isotope values and the presence and abundance of parasites in treated mesocosms as compared to three control ones. Incorporation of the nutrient enrichment in the food web was seen as increased nutrient concentrations in the epilimnion and as a decrease in carbon:nutrient ratios and δ¹⁵N values in Daphnia. Nutrient enrichment did not significantly influence algal, bacterial or Daphnia biomass. One of the four parasite species observed, unidentified small gut parasite, had a higher prevalence (percentage of Daphnia infected) in treated mesocosms, but its intensity (number of parasites per infected host) remained the same among treatments. Our results suggest that the effect of nutrient enrichment on host–parasite dynamics is dependent on complex interactions within food webs and on the epidemiological traits of parasites.     

Variability of water temperature may influence food-chain length in temperate streams

Nutrient enrichment may alter population dynamics of species in different ways depending on their life strategies. The aim of this study was to test the effect of different nutrient concentrations on the population development of two bacterivorous freshwater nematodes, Bursilla monhystera and Plectus aquatilis. Microcosms with autoclaved natural sand from a pristine stream (Fuirosos, NE of Spain) were enriched with different levels of phosphate, nitrate and ammonia as inorganic nutrients and glucose as a biodegradable dissolved organic carbon source. Although leaching of carbon and nutrients from the detritus fraction in the sediment initially may have overruled differences between treatments, later samplings revealed bottom-up control, with Bursilla monhystera abundances positively correlated to bacterial abundances at high nutrient concentrations. Nevertheless, there were several indications that nematodes in turn affected microbial abundance, most likely through excretion of ammonia and through grazing. In contrast to B. monhystera, Plectus aquatilis at high nutrient concentrations showed a unimodal abundance curve, while not increasing in abundance at low nutrient concentrations. Glucose enrichment did not have any stimulatory effect on either microbial or nematode abundances, probably as a result of unfavourable C:N:P stoichiometry. P enrichment, by contrast, stimulated microbial and Bursilla abundances. Our results indicate that episodic nutrient enrichment may affect populations of bacterial-feeding nematodes in the short term. Their longer-term dynamics may, however, be more dependent on leaching of carbon and nutrients from the pools of sediment-bound detritus.     

Timing of disturbance,top‐down,and bottom‐up driving on early algal succession patterns in a tropical intertidal community

Herbivory and nutrient enrichment are major drivers of the dynamics of algal communities. However, their effects on algal abundance are under the influence of seasons. This study investigated the effects of herbivory and nutrient enrichment on early algal succession patterns using cages (uncaged and fully caged treatments) and two nutrient levels (ambient and enriched concentrations). To determine seasonal influences, experiment plots on dead coral patches were cleared during both dry and rainy season. Of the 17 algal species recruited in the experiment plots, three were dominant: Ulva paradoxa C. Agardh, Padina in the Vaughaniella stage, and Polysiphonia sphaerocarpa Børgesen. In this succession process, U. paradoxa was the earliest colonizer and occupied the cleared plots within the first month after clearing with the highest percentage of 83.33 ± 1.67% to 88.33 ± 9.28%. Then, it was replaced by the late successional algae, Padina in the Vaughaniella stage, and P. sphaerocarpa. The effects of herbivory and nutrient enrichment on algal abundance varied across algal functional groups and seasons. During the dry season, neither herbivory nor nutrient enrichment affected Ulva cover but during the rainy season, Ulva cover was influenced by nutrient enrichment. However, the abundance of algae in this early stage was not apparently affected by either herbivory or nutrient enrichment. Our results indicated that the timing of disturbance strongly influenced the algal abundance and successional patterns in this tropical intertidal community.     

Community structure and functional roles of meiofauna in the North Sea

Knowiedge on community structure of North Sea meiofauna has greatly increased recently. A quasisynoptic picture of meiofauna densities and copepod community structure from 171 stations of the southern North Sea, sampled in April–May 1986, has been obtained during the North Sea Benthos Survey. Latitudinal patterns in meiofauna abundance and copepod weight, abundance and diversity exist in an area between 51°30′N and 58°30′N. Using TWINSPAN-classification five major groups of copepod species can be recognized which are related to sediment type, latitude and depth. The part of the meiofauna in total benthic energy flow, their role in the benthic food web and in biogeochemical cycles is discussed based on existing literature. There are still considerable gaps in knowledge and the field is not progressing rapidly. Publication no. 599 Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, Yerseke, The Netherlands.     

Responses of benthic algae to nutrient enrichment in a shallow lake: Linking community production,biomass, and composition

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Microbial–meiofaunal interrelationships in coastal sediments of the Red Sea

Population density and biomass of bacteria and meiofauna were investigated seasonally in the sediments of the north-western bank of Red Sea. Samples of sediments were collected seasonally from three different stations to determine microphytobenthic biomass (chlorophyll a), protein, lipid, carbohydrate, and total organic matter concentrations. These investigations revealed that microbial components tended to increase their dominancy, whereas sensitive meiofauna were extremely reduced during the entire study period. Thus a very low density of the total meiofauna (with an annual average of 109 ± 26 ind./10 cm²) was recorded whilst the benthic microbial population densities exhibited higher values (ranging from 0.31 ± 0.02 × 10⁸ to 43.67 ± 18.62 × 10⁸/g dry sediment). These changes in the relative importance analysis of benthic microbial components versus meiofaunal ones seem to be based on the impact of organic matter accumulation on the function and structure of these benthic communities. Proteins, lipids and carbohydrates showed very low concentration values, and the organic matter mostly consisted of carbohydrates, reflecting lower nutritional values for benthic fauna in general and meiofauna in particular. The distribution of microbial and meiofaunal communities seems to be dependent on the quality of the organic matter rather than on its quantity. Total organic matter concentrations varied between 5.8 and 7.6 mg/g, with organic carbon accounting for only 32% of the total organic matter. Chlorophyll a attained very low values, fluctuating between 0.11 and 0.56 μg/g, indicating the oligotrophy of the studied area. The very low concentration of chlorophyll a in the Red Sea sediment suggests that the sedimentary organic matter, heterotrophic bacteria and/or protozoa constitute an alternative resource that is consumed by meiofauna when algae are less abundant. Protozoa, therefore, represent the “missing link in bacteria–meiofauna interaction in the Red Sea marine sediment ecosystem.     

Effects of flow on meiofauna colonization in artificial streams and reference sites within the Illinois River, Arkansas

Despite frequent disturbances from flow, stream meiofauna form diverse and abundant assemblages suggesting that they are resistant and/or resilient to flow disturbances. Stream flow profoundly influences benthic invertebrate communities but these effects remain poorly understood. We examined the influence of flow on meiofauna colonization at small spatial scales (2–3 m) using artificial streams in conjunction with similar sites (flow, depth, substrates) in the reference stream (Illinois River, Arkansas). Colonization of meiofauna was found to be rapid and generally increased with flow rates examined (1–2, 6–7, and 11–12 cm s⁻¹). Six of the 10 most abundant taxa successfully completed colonization in artificial channels (equaled or exceeded reference benthic densities) within 5 days. Benthic meiofauna were more abundant in fast flows in artificial channels and in fast and slow flows in reference stream sites. A diverse assemblage of meiofauna was collected from the plankton which was dominated by rotifers, copepods (mostly nauplii), dipterans, and cladocerans. Densities of drifting meiofauna (potential colonists of the benthos) were low (5 no. l⁻¹) and similar among artificial channels and reference sites regardless of flow rates (F _1,18 = 2.19, p = 0.1407). Although densities were low, the numbers of drifting meiofauna were more than sufficient to colonize the benthos. Less than 0.65% of the drifting meiofauna were needed to colonize the substrates of artificial streams. The benthic assemblage paralleled that of the plankton, consisting mainly of rotifers, copepods (mainly nauplii), and dipterans. Evidence for active control over dispersal was observed as meiofauna densities varied between the plankton and benthos over the diel cycle (F _1,18 = 6.02, p = 0.0001 and F _1,18 = 9.88, p = 0.006, respectively). Rotifers, copepods, and nematodes were more abundant in the plankton during the day and in the substrates at night. These results suggest that meiofauna assemblages can change rapidly in response to alterations of habitat patches by disturbance.     

Meiofauna and macrofauna communities in a mangrove from the Island of Santa Catarina, South Brazil

The composition and abundance of the meiofauna and macrofauna were studied in a survey carried out within 6 locations in a mangrove at the Island of Santa Catarina, South Brazil. Nine meiofaunal taxa were registered with densities ranging between 77 and 1589 inds.10 cm^?2. The nematodes, composed by 94 putative species (86 genera), largely dominated the meiofauna. The most abundant genera were Haliplectus (Haliplectidae), Anoplostoma (Anoplostomatidae) and Terschillingia (Linhomoidae). Contrary to the meiofauna, the macrofauna showed a low number of taxa (only 17 recorded) and abundance (up 7250 inds.m^?2). The macrofauna was mainly composed by deposit feeders, and numerically dominated by oligochaetes and capitellid polychaetes. For both components, differences in the composition and abundance along the sampling sites were significant but not primarily related to the typical variations along estuaries, such as salinity. The results of the stepwise multiple regression analyses showed that the detritus biomass (ash-free dry weight) was the most important predictor of faunal densities and diversity. The clear relationship between detritus and fauna, together with the contrasting community structure of the two component of the benthos suggest that the meiofauna showed a high efficiency in exploiting the micro-habitat created by the presence of the detritus. Yet the macrofauna, potentially the main consumer of the debris, is negatively affected by their low palatability and poor nutritive value.     

Nutrient limitation and algal blooms in urbanizing tidal creeks

Tidal creeks are commonly found in low energy systems on the East and Gulf Coasts of the United States, and are often subject to intense watershed human development. Many of these creeks are receiving urban and suburban runoff containing nutrients, among other pollutants. During the period 1993-2001, we studied three tidal creeks located in southeastern North Carolina, a rapidly urbanizing area. All three creeks received anthropogenic nutrient loading. Oligohaline to mesohaline stations in upper tidal creek regions had much higher nutrient (especially nitrate-N) concentrations than lower creek areas, and hosted spring and summer phytoplankton blooms that at times exceeded 200 μg chlorophyll a l⁻¹. Phytoplankton biomass during winter was low at all stations in all three creeks. Spring and summer nutrient addition bioassay experiments were conducted to characterize the nutrients limiting phytoplankton growth. Water from high salinity stations in all three creeks always showed significant positive responses to nitrate-N inputs, even at concentrations as low as 50 μg N l⁻¹. Low salinity stations in upper creek areas often showed significant responses to nitrate-N inputs, but on occasion showed sensitivity to phosphorus inputs as well, indicating the influence of anthropogenic nitrate loading. During several experiments, one of the upper stations showed no positive response to nutrient inputs, indicating that these stretches were nutrient replete, and further phytoplankton growth appeared to be light-limited either by phytoplankton self-shading or turbidity. Water from upper creek areas yielded much higher chlorophyll a concentrations in bioassay experiments than did lower creek water. In general, these urbanizing tidal creeks were shown to be very sensitive to nitrogen loading, and provide a physical environment conducive to phytoplankton bloom formation in nutrient-enriched areas. Tidal creeks are important ecological resources in that they are considered to be nursery areas for many species of fish and shellfish. To protect the ecological function of these small, but very abundant estuarine systems, management efforts should recognize their susceptibility to algal blooms and focus on control of nonpoint source nutrient inputs, especially nitrogen.     

Control mechanisms of arctic lake ecosystems: a limnocorral experiment

To assess the potential impact of human exploitation on arctic lakes and to determine how these eco systems are regulated we initated a limnocorral experiment in Toolik Lake, Alaska, in the summer of 1983. The limnocorrals were 5 m in diameter and from 5–6 m in depth and were open to the sediments. In 1983 four limnocorrals were deployed in an isolated bay of Toolik Lake within a cross-classified treatment regime of high and low inorganic nitrogen and phosphorus additions and high and low free swimming fish additions. The objective of the nutrient addition was to stimulate phytoplankton growth and determine the extent to which increased plant production was passed through pelagic and benthic food chains. The objective of the fish addition was to determine the impact of fish predation on large-bodied zooplankton, especially the zooplanktivorous copepod Heterocope, then to study the effect of altered Heterocope densities on small-bodied zooplankton species population dynamics. In 1984 two more limnocorrals were deployed, one a low fish, 1 × nutrient addition treatment and the other a no fish, no nutrient treatment. The fish manipulation was changed to confining several fish in cages with the cages held in corrals for varying lengths of time. The addition of inorganic nitrogen and phosphorus dramatically increased phytoplankton productivity. This increase in algal biomass and production greatly altered the light environment and water quality in the nutrient treated limnocorrals. The secchi disk depth in the nutrient treated limnocorrals declined each summer reaching as low as 1 m in 1985. Both oxygen content and pH increased in the nutrient treatment corrals. Corrals not receiving nutrient additions remained near lake concentrations for most water quality parameters. While phytoplankton biomass was stimulated in 1983 phytoplankton growth was not sufficient to draw down all the nitrogen and phosphorus added and these nutrients reached high levels in the last half of the summer. In 1984 phosphorus remained above 20 μg in the nutrient-treated corrals but ammonia dropped to reference levels by day 25. In 1985 both nutrient concentrations rapidly declined to reference levels. Most pelagic components responded to the nutrient additions. Microbial production was stimulated in the nutrient treated limnocorrals and bacterial population sizes built up to nearly 8–10 times those of the reference corrals. However, microheterotrophs soon increased in abundance and apparently grazed down bacteria to reference levels. Phytoplankton population density, as estimated by chlorophyll a determinations, increased dramatically with nutrient addition such that each year the phytoplankton densities were higher than before. Primary productivity was also stimulated and appeared not to be light limited even when phytoplankton densities rose to high levels. In the first two years of the experiment zooplankton densities were little altered by the increased phytoplankton densities. However, by 1985 daphnid densities were quite a bit higher in the high nutrient addition limnocorrals. The benthic community and sediment response was much less affected by nutrient addition. Overall sediment respiration increased in the nutrient treated corrals but underlying sediments seemed little affected. Decomposition of Carex litter was likewise little affected by nutrient addition. Benthic invertebrates were also little impacted by the nutrient addition and increased sedimentation of phytoplankton. However, the response of benthic invertebrates is difficult to assess fully in the current experiment because chironomids, a prominent component of the benthic community, failed to recruit into the limnocorrals and the corrals physically shifted during ice-out in the spring of 1984 disturbing the sediment in several corrals. The fish additions in 1983 of free swimming grayling essentially eliminated large bodied zooplankton, especially Heterocope septentrionalis, from all four limnocorrals. In subsequent summers Heterocope were not so dramatically preyed upon but generally were found in higher densities in the low or no fish treatments. However, either when Heterocope were eliminated in 1983 or were in rough inverse proportion to fish density, altered Heterocope abundance had no obvious affect on small-bodied zooplankton abundance. The fish treatment apparently influenced the zooplankton response to high nutrient addition in 1985. In the high nutrient limnocorrals daphnid populations became very abundant, but in the high fish treatment the daphnid responding was the small-bodied D. longiremis while in the low fish treatment the daphnid responding was the large-bodied D. middendorffiana. Thus we have considerable evidence for bottom up control of phytoplankton density and production. This increased production ultimately, but not for two years, stimulated zooplankton density increases. Increased nutrients had little effect on the benthos or sediments. Fish manipulations influenced large-bodied zooplankton but had little effect on small-bodied zooplankton. Because grayling are predominantly plankton feeders in lakes, no fish effect on benthic invertebrates was expected. Limnocorrals thus seem good systems to study nutrient-phytoplankton interactions. They are not as suitable for benthic invertebrate studies and fish manipulations may be difficult. Most other limnocorral studies were of brief duration; however, in the present study the limnocorrals seemed to perform well over a three year period.