Local Nutrient Enrichment Research Articles

Latitudinal Patterns in European Seagrass Carbon Reserves: Influence of Seasonal Fluctuations versus Short-Term Stress and Disturbance Events.

Seagrass meadows form highly productive and valuable ecosystems in the marine environment. Throughout the year, seagrass meadows are exposed to abiotic and biotic variations linked to (i) seasonal fluctuations, (ii) short-term stress events such as, e.g., local nutrient enrichment, and (iii) small-scale disturbances such as, e.g., biomass removal by grazing. We hypothesized that short-term stress events and small-scale disturbances may affect seagrass chance for survival in temperate latitudes. To test this hypothesis we focused on seagrass carbon reserves in the form of starch stored seasonally in rhizomes, as these have been defined as a good indicator for winter survival. Twelve Zostera noltei meadows were monitored along a latitudinal gradient in Western Europe to firstly assess the seasonal change of their rhizomal starch content. Secondly, we tested the effects of nutrient enrichment and/or biomass removal on the corresponding starch content by using a short-term manipulative field experiment at a single latitude in the Netherlands. At the end of the growing season, we observed a weak but significant linear increase of starch content along the latitudinal gradient from south to north. This agrees with the contention that such reserves are essential for regrowth after winter, which is more severe in the north. In addition, we also observed a weak but significant positive relationship between starch content at the beginning of the growing season and past winter temperatures. This implies a lower regrowth potential after severe winters, due to diminished starch content at the beginning of the growing season. Short-term stress and disturbances may intensify these patterns, because our manipulative experiments show that when nutrient enrichment and biomass loss co-occurred at the end of the growing season, Z. noltei starch content declined. In temperate zones, the capacity of seagrasses to accumulate carbon reserves is expected to determine carbon-based regrowth after winter. Therefore, processes affecting those reserves might affect seagrass resilience. With increasing human pressure on coastal systems, short- and small-scale stress events are expected to become more frequent, threatening the resilience of seagrass ecosystems, particularly at higher latitudes, where populations tend to have an annual cycle highly dependent on their storage capacity.

Seasonal phytoplankton blooms in the Gulf of Aden revealed by remote sensing

The Gulf of Aden, situated in the northwest Arabian Sea and linked to the Red Sea, is a relatively unexplored ecosystem. Understanding of large-scale biological dynamics is limited by the lack of adequate datasets. In this study, we analyse 15years of remotely-sensed chlorophyll-a data (Chl-a, an index of phytoplankton biomass) acquired from the Ocean Colour Climate Change Initiative (OC-CCI) of the European Space Agency (ESA). The improved spatial coverage of OC-CCI data in the Gulf of Aden allows, for the first time, an investigation into the full seasonal succession of phytoplankton biomass. Analysis of indices of phytoplankton phenology (bloom timing) reveals distinct phytoplankton growth periods in different parts of the gulf: a large peak during August (mid-summer) in the western part of the gulf, and a smaller peak during November (mid-autumn) in the lower central gulf and along the southern coastline. The summer bloom develops rapidly at the beginning of July, and its peak is approximately three times higher than that of the autumnal bloom. Remotely-sensed sea-surface temperature (SST), wind-stress curl, vertical nutrient profiles and geostrophic currents inferred from the sea-level anomaly, were analysed to examine the underlying physical mechanisms that control phytoplankton growth. During summer, the prevailing southwesterlies cause upwelling along the northern coastline of the gulf (Yemen), leading to an increase in nutrient availability and enhancing phytoplankton growth along the coastline and in the western part of the gulf. In contrast, in the central region of the gulf, lowest concentrations of Chl-a are observed during summer, due to strong downwelling caused by a mesoscale anticyclonic eddy. During autumn, the prevailing northeasterlies enable upwelling along the southern coastline (Somalia) causing local nutrient enrichment in the euphotic zone, leading to higher levels of phytoplankton biomass along the coastline and in the lower central gulf. The monsoon wind reversal is shown to play a key role in controlling phytoplankton growth in different regions of the Gulf of Aden.

Even moderate nutrient enrichment negatively adds up to global climate change effects on a habitat-forming seaweed system

Coastal marine ecosystems have been under high anthropogenic pressure and it can be assumed that prevalent local perturbation interacts with rising global stressors under proceeding climate change. Understanding their effective pathways and cumulative effects is of high relevance not only with regard to future risk assessment, but also for current ecosystem management. In benthic mesocosms, we factorially tested the effects of one global (combined elevated seawater temperature and CO2 concentration) and one local (nutrient enrichment) stressor on a common coastal Baltic seaweed system (Fucus vesiculosus). Both treatments in combination had additive negative impacts on the seaweed—epiphyte—mesograzer system by altering its regulatory mechanisms. That is, warming decreased the biomass of two mesograzer species (weakened top-down control), whereas moderate nutrient enrichment increased epiphyte biomass (intensified bottom-up control), which ultimately resulted in a significant biomass reduction of the foundation seaweed. Our results suggest that climate change impacts might be underestimated if local pressures are disregarded. Furthermore, they give implication for local ecological management as the mitigation of local perturbation may limit climate change impacts on marine ecosystems.

Effects of point source of untreated sewage waters on seagrass (Zostera marina and Z. noltii) beds in the South-Western Black Sea

Tourist development is rapid on the South-West coast of the Black Sea, increasing the pressure on the coastal ecosystem. Zostera marina and Zostera noltii are abundant seagrasses in the Black Sea, but little is known about their sensitivity towards coastal developments. In this study plant nutrient content (N and P), sulphide intrusion, sediment organic matter, pore water pools of nutrients and sulphides, sulphate reduction rates as well as water column nutrients and chlorophyll-a concentrations were studied in a mixed seagrass meadow (Z. marina L. and Z. noltii Hornem.) located in an area receiving untreated sewage waters from a point source in Sozopol Bay, South-Western Black Sea. In addition, studies included population level metrics like density, biomass and shoot morphology. The point source impacted water quality in the area leading to local nutrient enrichment, development of filamentous macroalgae and reduced water transparency. Although sulphide intrusion was detected in Z. marina and thus indicating existence of sulphide pressure on the plants, there were no significant effects on the seagrass distribution except for Z. noltii being absent from the meadows close to the point source. The nutrient levels were low in the plants and in the sediments compared to global Z. marina and Z. noltii meadows, but the observed blooms of filamentous macroalgae, increase in sulphide intrusion and higher N contents of the plants during the peak tourist season are signs of eutrophication in Sozopol Bay most likely as a result of increased recreational activity on the coast.

Examining 239+240Pu, 210Pb and historical events to determine carbon, nitrogen and phosphorus burial in mangrove sediments of Moreton Bay, Australia

Two sediment cores were collected in a mangrove forest to construct geochronologies for the previous century using natural and anthropogenic radionuclide tracers. Both sediment cores were dated using 239+240Pu global fallout signatures as well as 210Pb, applying both the Constant Initial Concentration (CIC) and the Constant Rate of Supply (CRS) models. The 239+240Pu and CIC model are interpreted as having comparable sediment accretion rates (SAR) below an apparent mixed region in the upper ∼5 to 10 cm. In contrast, the CRS dating method shows high sediment accretion rates in the uppermost intervals, which is substantially reduced over the lower intervals of the 100-year record. A local anthropogenic nutrient signal is reflected in the high total phosphorus (TP) concentration in younger sediments. The carbon/nitrogen molar ratios and δ15N values further support a local anthropogenic nutrient enrichment signal. The origin of these signals is likely the treated sewage discharge to Moreton Bay which began in the early 1970s. While the 239+240Pu and CIC models can only produce rates averaged over the intervals of interest within the profile, the 210Pb CRS model identifies elevated rates of sediment accretion, organic carbon (OC), nitrogen (N), and TP burial from 2000 to 2013. From 1920 to 2000, the three dating methods provide similar OC, N and TP burial rates, ∼150, 10 and 2 g m−2 year−1, respectively, which are comparable to global averages.

Interactions between megaherbivores and microherbivores: elephant browsing reduces host plant quality for caterpillars

Direct effects of herbivory, and indirect effects through induced responses to herbivory, can both influence the susceptibility of plants to subsequent attacks by herbivores. There has, however, been very little research (if any) to investigate how the large‐scale effects of browsing by megaherbivores (>1000 kg body mass) on woody plants might influence the subsequent use of those plants by phytophagous insects. We conducted a field study in Kruger National Park, South Africa, to investigate whether browsing by elephants (Loxodonta africana) on mopane trees (Colophospermum mopane) had any influence on the subsequent selection of those trees by ovipositing mopane moths (Imbrasia belina). Our results showed that, after controlling for differences in canopy volume, the density of egg masses was almost halved in mopane woodlands recovering from severe elephant browsing in the previous season. This is despite the regrowth on heavily browsed trees having lower tannin:protein ratios and longer shoots. Our suggested explanation is that large monophagous caterpillars can only feed in the canopies of the trees in which they hatch and so the quantity of food in each canopy is more important than its quality. There are implications for the sustainable harvesting of mopane caterpillars, which represent an important food resource for rural communities in southern Africa. Future avenues for research include patch selection by large herbivores in response to local nutrient enrichment by frass deposited during caterpillar outbreaks.

Positive effect of shredders on microbial biomass and decomposition in stream microcosms

Summary1. Animals play a major role in nutrient cycling via excretory processes. Although the positive indirect effects of grazers on periphytic algae are well understood, little is known about top‐down effects on decomposers of shredders living on leaf litter.2. Nutrient cycling by shredders in oligotrophic forest streams may be important for the microbial‐detritus compartment at very small spatial scales (i.e. within the leaf packs in which shredders feed). We hypothesised that insect excretion may cause local nutrient enrichment, so that microorganism growth on leaves is stimulated.3. We first tested the effect of increasing concentration of ammonium (+10, +20 and +40 μg NH4+ L−1) on fungal and bacterial biomass on leaf litter in a laboratory experiment. Then we performed two experiments to test the effect of the presence and feeding activity of shredder larvae. We used two species belonging to the trichopteran family Sericostomatidae: the Palaearctic Sericostoma vittatum and the Neotropical Myothrichia murina, to test the effect of these shredders on fungal and bacterial biomass and decomposition on leaves of Quercus robur and Nothofagus pumilio, respectively. All experiments were run in water with low ammonium concentrations (2.4 ± 0.34 to 14.47 ± 0.95 μg NH4+ L−1).4. After 5 days of incubation, NH4 concentrations were reduced to near‐ambient streamwater concentrations in all treatments with leaves. Fungal biomass was positively affected by increased ammonium concentration. On the other hand, bacteria abundance was similar in all treatments, both in terms of abundance (bacteria cells mg−1 leaf DW) and biomass. However, there was a tendency towards larger mean cell size in treatments with 20 μg NH4 L−1.5. In the experiment with S. vittatum, fungal biomass in the treatment with insects was more than twice that in the control after 15 days. Bacteria were not detected in treatments with insects, where hyphae were abundant, but they were abundant in treatments without larvae. In the decomposition experiment run with M. murina, leaf‐mass loss was significantly higher in treatments with larvae than in controls.6. Our hypothesis of a positive effect of shredders on fungal biomass and decomposition was demonstrated. Insect excretion caused ammonium concentration to increase in the microcosms, contributing to microbial N uptake in leaf substrata, which resulted in structural and functional changes in community attributes. The positive effect of detritivores on microbes has been mostly neglected in stream nutrient‐cycling models; our findings suggest that this phenomenon may be of greater importance than expected in stream nutrient budgets.

Ant nests and soil nutrient availability: the negative impact of fire

Abstract:High-nutrient patches are important for regulating the structure and physiognomy of dystrophic habitats. Leaf-cutting ants create these rich patches in many neotropical habitats. Burning, however, could diminish or even annul the effects of ant nests on soil properties. To test this hypothesis, we compared the nutrient concentrations at various depths in soil samples near three nests of the leaf-cutting ant Atta laevigata and three non-nest soils, located in 10-ha plots subjected to burning or with fire-protection within a Brazilian cerrado. Root density in ant-nest soil was greater than in non-nest soil in both unburned and burned plots. Besides, the concentration of Ca, Mg, K and P increased 2–50-fold in ant nests from the unburned area compared to non-nest soils. In contrast, nutrient concentrations and pH in burned ant nests were similar to or lower than non-nest soils, but the nests maintained higher values of organic matter and cation exchange capacity. The positive effect of leaf-cutting ant nests as high-nutrient patches may be strongly reduced in habitats with frequent burning. In this case, the negative effects of leaf-cutter herbivory on a plant community could surpass the benefits of local nutrient enrichment by their nests.Resumo: Sauveiros modificam as propriedades físicas e químicas do solo, alterando a estrutura e fisionomia local. Entretanto, queimadas podem reduzir os efeitos dos formigueiros nesses ambientes como, por exemplo, pela diminuição do forrageamento das formigas e, conseqüentemente, pela baixa incorporação de material orgânico ao solo. Este estudo testou o efeito do fogo sobre a concentração de nutrientes no solo de colônias de Atta laevigata em área de cerrado. A densidade de raízes foi maior nas colônias do que áreas controle, independentemente da presença do fogo. Na ausência de queimadas, as concentrações de nutrientes foram até 50 vezes superiores nos sauveiros. Contudo, queimadas reduzem a concentração de nutrientes e pH nas colônias. Possivelmente, essa baixa concentração seja atribuída à maior demanda pela vegetação remanescente, em resposta à perturbação provocada pelo fogo. Nesse caso, os efeitos negativos da herbivoria podem superar os benefícios causados por meio da disponibilização de nutrientes pelos sauveiros.

Seasonal variation of riverine nutrient inputs in the northern Bay of Biscay (France), and patterns of marine phytoplankton response

Seasonal variations in nutrient inputs are described for the main rivers (Loire and Vilaine) flowing into the northern Bay of Biscay. The river plumes are high in N/P ratio in late winter and spring, but not in the inner plume during the summer. Conservative behavior results in most nutrients entering the estuary and eventually reaching the coastal zone. Temporal and spatial aspects of phytoplankton growth and nutrient uptake in the northern Bay of Biscay distinguish the central area of salinity 34 from the plume area. The first diatom bloom appears offshore in late winter, at the edge of the river plumes, taking advantage of haline stratification and anticyclonic “weather windows.” In spring, when the central area of the northern shelf is phosphorus-limited, small cells predominate in the phytoplankton community and compete with bacteria for both mineral and organic phosphorus. At that period, river plumes are less extensive than in winter, but local nutrient enrichment at the river mouth allows diatom growth. In summer, phytoplankton become nitrogen-limited in the river plumes; the central area of the shelf is occupied by small forms of phytoplankton, which are located on the thermocline and use predominantly regenerated nutrients.

Daily carbon, nitrogen and phosphorus budgets for the Mediterranean planktivorous damselfish Chromis chromis

The planktivorous damselfish Chromis chromis is one of the most numerous fish species in the Mediterranean rocky littoral, yet the role this species plays in the overall functioning of the ecosystem is poorly understood. In this study nitrogen, carbon and phosphorus budgets were constructed for C. chromis from the Bay of Calvi, Corsica and an attempt was made to resolve the possible fates of these nutrients from ingested materials, to somatic growth ( P s), reproductive tissues ( P r), respiration ( R), assimilation ( A′), faeces ( F) and dissolved wastes ( U). C. chromis were estimated to consume on average 4.7% of their own body mass of zooplankton per day, in two separate feeding episodes — at dawn and at dusk. This is equivalent to 8288 individual copepods or 42.1 mg carbon, 9.7 mg nitrogen, 0.8 g phosphorus. 44% of all phosphorus ingested was shown to be excreted as dissolved waste, and 46% as faeces; this may be of considerable importance in the Mediterranean, where littoral macrophytes are often severely phosphorus-limited. 85% of all nitrogen was excreted as dissolved waste (mostly ammonia) and local nutrient enrichment may enhance littoral primary productivity, particularly around C. chromis night-shelters.

Feeding and growth of the isopod Idotea baltica on the brown alga Fucus vesiculosus: Roles of inter-population and within-plant variation in plant quality

: We investigated the effects of local nutrient enrichment, among-population variation in quality, and within-plant variation in quality on growth, development, and food consumption (total amount of food ingested) of the marine isopod Idotea baltica when fed Fucus vesiculosus. We found that the proximity of fish farms increased carbon/nitrogen ratio, but did not change the concentration of phlorotannins in the F. vesiculosus thallus. Algal diets taken from sites close to or far from fish farms did not affect the growth, development, or total amount of food ingested of I. baltica. Therefore, eutrophication may not necessarily decrease the level of plant resistance. We found that different F. vesiculosus beds vary in quality and led to differences in total amount of food ingested and production efficiency of I. baltica. The variation in quality of F. vesiculosus in different locations may generate variance in grazing pressure between the belts. Idotea baltica was most sensitive to within-plant variation, as the isopods grew more and had higher production efficiency on apical than basal parts of F. vesiculosus. Idotea baltica did not compensate for the lower quality of basal parts by increasing consumption. This may indicate that the low quality of the basal thallus is related to higher levels of secondary metabolites and not to lower levels of soluble sugars or nitrogen that might have encouraged compensatory consumption. Our results suggest that within-plant variation in the quality of the F. vesiculosus is more important for herbivores than possible environmental variation caused by eutrophication.

Influence of a Gulf Stream warm-core ring on water mass and chlorophyll distributions along the southern flank of Georges Bank

During late spring 1997, a Gulf Stream warm-core ring (WCR) strongly influenced water mass and chlorophyll distributions along the southern flank of Georges Bank. Entrainment of Georges Bank shelf water by the WCR, centered at the central southern flank, persisted through May. By late May, shelf water encircled nearly the entire WCR, and horizontal convergence toward the central southern flank was evident. Scotian Shelf water (SSW) extended across Northeast Channel, onto Georges Bank near the northeast peak, and along the southern flank between the 60-m isobath and the shelfbreak front. SSW extended furthest southwest along the southern flank shelfbreak. Satellite imagery from the ocean color and temperature sensor (OCTS) showed that by late May, in addition to the high chlorophyll concentrations within the entrained shelf water encircling the WCR, pigment rich bands of chlorophyll developed along the 60- and 100-m isobaths. Where this biological enhancement developed along >100 km of the shelfbreak (100-m isobath), shelf waters extended furthest seaward due to entrainment by the WCR. The enhanced shelfbreak chlorophyll was sampled in situ along two transects separated by ≈40 km along shelf. Along both transects, the enhanced chlorophyll coincided with divergent cross-shelf flow, maximum along-shelf flow and minimum surface temperature. Coincidence of the highest surface chlorophyll and lowest surface temperature with divergent cross-shelf flow is consistent with upwelling. Coincidence with along-shelf jets and their associated vertical shear is consistent with turbulent vertical mixing. The importance of turbulent vertical mixing was supported. Maximum velocity at the shelfbreak coincided with the subsurface temperature minimum of SSW near 30-m depth. Estimated gradient Richardson numbers (Ri) above the SSW were below critical (<0.25) within the shelfbreak chlorophyll maximum along both transects, and surface chlorophyll was significantly, inversely correlated with Ri along the southern flank. WCR entrainment of shelf water continued into June as the WCR propagated southwestward. On June 11, high chlorophyll concentrations along the shelfbreak north of the WCR were observed by the OCTS. Satellite sea-surface temperature showed that in addition to WCR entrainment, breaking waves ( λ≈30 km) of the shelf-slope front coincided directly with the locally enhanced chlorophyll. The waves propagated west at ≈12 km day −1 . Vertical mixing due to these breaking frontal waves may have contributed to local nutrient enrichment of near-surface waters at the shelfbreak.

Interactions between nutrient status and weevil herbivory in the biological control of water hyacinth

Summary 1. Despite the widespread release of effective biocontrol agents, water hyacinth remains the world’s most problematic aquatic weed, particularly in eutrophic waterbodies. However, understanding of the interaction between control and trophic status is still incomplete. 2. Growth of water hyacinth plants was measured at two water nutrient concentrations (high and medium) and in the presence and absence of two insect biocontrol agents in a large circulating hydroponic system in a glasshouse. 3. At the high nutrient concentration (1·6 mg l−1 N and 1·0 mg l−1 P), plants multiplied more quickly, attaining greater biomass. Both insect species reduced plant growth at both nutrient concentrations. Neochetina bruchi, however, performed better than N. eichhorniae at the high nutrient concentration by inflicting more damage on the plants and reducing biomass by a greater extent. 4. Insect damage reduced the concentrations of nitrogen and phosphorus found in plants growing in high nutrient water but not in medium nutrient water (0·4 mg l−1 N and 0·025 mg l−1 P). 5. The developmental and reproductive performance of N. bruchi was determined at both nutrient concentrations. Water hyacinth plants grown at higher nutrient concentration were superior hosts to N. bruchi than plants grown at medium concentrations. Net reproductive rate and intrinsic rate of increase were significantly greater at the high concentrations. Greater damage by N. bruchi to water hyacinth at the high nutrient concentration was due to the greater production of offspring, and hence greater larval damage. 6. We predict that water hyacinth problems will be greater in eutrophic waterbodies, where N. bruchi will be a superior biocontrol agent to N. eichhorniae. In low‐nutrient waterbodies, local nutrient enrichment of water may assist the establishment of control agents. These results illustrate the importance of wider ecological factors on the success of biological control.

Chlorophyll enhancement and mixing associated with meanders of the shelf break front in the Mid‐Atlantic Bight

Meanders of the shelf break front in the Mid‐Atlantic Bight (MAB) during April and May of 1997 were associated with chlorophyll enhancement along a hydrographic and a topographic feature. The hydrographic feature was the surface outcrop of the front, which ranged from ∼10 to &gt;100 km seaward of the shelf break owing to the meanders. The topographic feature was the shelf break (100‐m isobath). Chlorophyll enhancement was observed by a satellite instrument, the ocean color and temperature sensor, and by a fluorometer in situ. It developed in near‐surface waters typically nutrient depleted during late spring, thus local nutrient enrichment of near‐surface waters was probable. Observations of sufficient resolution to define processes were available only for the region of shelf break chlorophyll enhancement. Along two meander troughs (shoreward extremities near the shelf break), we observed shoaling of cold shelf water. Shelf water shoaled &gt;20 m along frontal isopycnals, and phytoplankton absorption maxima coincided directly with the shoaled water. Thus local nutrient enrichment by along‐isopycnal upwelling was the supported mechanism of chlorophyll enhancement at the shelf break. The basis for along‐isopycnal upwelling was seaward flow of shelf water forced by meander circulation near the shelf break. Strong cross‐isobath flow and mixing developed as these meanders propagated along the shelf break front of the MAB at a relatively constant rate of ∼9 km day−1.

Root morphological plasticity and nutrient acquisition of perennial grass species from habitats of different nutrient availability.

We studied the root foraging ability and its consequences for the nutrient acquisition of five grass species that differ in relative growth rate and that occur in habitats that differ widely in nutrient availability. Foraging responses were quantified, based on the performance of the plants in homogeneous and heterogeneous soil environments of the same overall nutrient availability. Although all species tended to produce a significantly higher root length density in a nutrient-rich patch, this response was significant only for the faster-growing species. The increased root length density resulted from small, though not significant, changes in root biomass and specific root length. The effectiveness of root proliferation was determined by quantifying the total amount of nutrients (N and P) accumulated by the plants over the course of the experiment. Plants acquired more N in a heterogeneous environment than in a homogeneous environment, although the total nutrient availability was the same. The ability to acquire nutrients (N or P) in the heterogeneous environment was not related to the ability of species to increase root length density in response to local nutrient enrichment. In contrast to other studies, our results suggest that the role of morphological plasticity of roots in acquiring patchily distributed resources is limited. Possible reasons for this discrepancy are discussed.

Responses of Picea, Pinus and Pseudotsuga roots to heterogeneous nutrient distribution in soil.

The spatial distribution of plant-available mineral nutrients in forest soils is often highly heterogeneous. To test the hypothesis that local nutrient enrichment of soil leads to increased root proliferation in the nutrient-rich soil zone, we studied the effects of nutrient enrichment on the growth and nutrient concentrations of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) roots. Three-year-old seedlings were grown for 9 months in split-root containers filled with nutrient-poor forest mineral soil, with one side supplemented with additional mineral nutrients. Root dry weight and root length in Scots pine and Norway spruce were increased in the nutrient-supplemented soil compared with the nonsupplemented side, whereas root growth in Douglas-fir was unaffected by nutrient enrichment. Of the three species examined, Norway spruce exhibited the highest root and shoot growth and the highest nutrient demand. Specific root length (m g(-1)) and the number of root tips per unit root length were not affected by local nutrient addition in any of the species. Despite increased root growth in Norway spruce and Scots pine in nutrient-supplemented soil, their root systems contained similar nutrient concentrations on both sides of the split-root container. Thus, coniferous trees may respond to local nutrient supply by increased root proliferation, but the response varies depending on the species, and may only occur when trees are nutrient deficient. As a response to local nutrient enrichment, increases in root dry matter or root length may be better indicators of pre-existing nutrient deficiencies in conifers than increases in root nutrient concentrations.

Direct and indirect effects of nutrients on intertidal community structure: variable consequences of seabird guano

The structure of communities on cliff sections with and without overhead seabird colonies was compared to assess whether local nutrient enrichment by birds affected the abundances of rocky-shore species in the San Juan archipelago of Washington state. In the presence of guano, the vertical distribution of orange lichens ( Caloplaca marina Wedd. and Xanthoria elegans (Link.) Th. Fr.) was elevated and gray lichens ( Physica (Schreb.) Michx.) were eliminated in the supralittoral fringe. In the splash and upper intertidal zones, guano appeared to directly enhance the abundance of the green alga Prasiola meridionalis S. & G. and fleshy crustose algae ( Mastocarpus papillatus (C. Ag.) Kutzing, Ralfsia pacifica Hollenb., and Hildenbrandia Nardo spp.), but to directly reduce the amount of area covered by Fucus distichus L., and Ulva L. spp. By directly affecting the abundance of these species, guano indirectly affected other species: Verrucaria mucosa Wahl. declined because of enhanced competition with Prasiola in the splash zone, and Balanus glandula Darwin declined because of increased desiccation in areas with reduced Fucus canopy. Overall, guano positively influenced only four of the eighteen taxa examined in the study, suggesting that, with the exception of species that live in unusual situations with low nutrient transport, macronutrients probably do not limit the abundance of species in this community.

EFFECTS OF CHIRONOMID (INSECTA: DIPTERA) TUBE‐BUILDING ACTIVITIES ON STREAM DIATOM COMMUNITIES1,2

ABSTRACTChironomid retreats, constructed out of sand grains upon submerged wood debris, increase the surface area available for diatom colonization. The three dimensional substratum afforded by chironomid tubes supports up to twelve times the diatom biavolume found upon adjacent, unmodified substratum in a northern Michigan stream. Diatom enumeration within scrapings from small defined areas on artificial substrata, combined with examination of intact natural substrata through scanning electron microscopy (SEM), reveals distinct, microdistribution patterns. The larval retreats of two major taxa of tube‐dwelling chironomid's (Micropsectra sp. and Pseudodiamesa cf. pertinax Garrett) display significantly different diatom communities relative to adjacent masonite substratum. Substratum without chironomid tubes is primarily colonized by Achnanthes minutissima Kütz. and Cocconeis placentula Ehr., exhibiting the lowest species diversity of microhabitats examined. The diatom flora upon sand tubes of Micropsectra sp. is dominated by Opephora martyi Herib., as is the flora of sand grains collected from the stream sediment load. These two micro‐habitats exhibit a high community similarity (SIMI). The SIMI index also suggests that the flora of P. pertinax tubes is highly similar to that of sand grains. Diversity, however, is almost three times greater on P. pertinax tubes and SEM observations reveal that this microhabitat is characterized by a more spatially complex flora; Nitzschia and Navicula spp. dominate the upperstory, and O. martyi is located on underlying sand grains. Results indicate that tube‐building chironomids in Carp Creek affect diatom microdistribution by: (1) stabilizing sand grains and associated flora within their retreats, (2) providing a ‘refugium’ for upperstory diatom taxa from the mayfly grazer, Baetis vagans McDunnough (Insecta: Ephemeroptera), and (3) through local nutrient enrichment.