The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms of N and P limitation.

/pdf/global-analysis-of-nitrogen-and-phosphorus-limitation-of-1j2o5ic67f.pdf

Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems.

The widespread occurrence of nitrogen limitation to net primary production in terrestrial and marine ecosystems is something of a puzzle; it would seem that nitrogen fixers should have a substantial competitive advantage wherever nitrogen is limiting, and that their activity in turn should reverse limitation. Nevertheless, there is substantial evidence that nitrogen limits net primary production much of the time in most terrestrial biomes and many marine ecosystems. We examine both how the biogeochemistry of the nitrogen cycle could cause limitation to develop, and how nitrogen limitation could persist as a consequence of processes that prevent or reduce nitrogen fixation. Biogeochemical mechansism that favor nitrogen limitation include: A number of mechanisms could keep nitrogen fixation from reversing nitrogen limitation. These include: The possible importance of these and other processes is discussed for a wide range of terrestrial, freshwater, and marine ecosystems.

Nitrogen limitation on land and in the sea: How can it occur?

https://www.owrb.ok.gov/quality/standards/pdf_standards/scenicrivers/Smith,%20Tilman%20and%20Nekola%201999.pdf

Eutrophication: impacts of excess nutrient inputs on freshwater, marine, and terrestrial ecosystems

A primary focus of coastal science during the past 3 decades has been the question: How does anthropogenic nutrient enrichment cause change in the structure or function of nearshore coastal ecosystems? This theme of environmental science is recent, so our conceptual model of the coastal eutrophication problem continues to change rapidly In this review, I suggest that the early (Phase I) con- ceptual model was strongly influenced by limnologists, who began intense study of lake eutrophication by the 1960s The Phase I model emphasized changing nutrient input as a signal, and responses to that signal as increased phytoplankton biomass and primary production, decomposition of phytoplankton- derived organic matter, and enhanced depletion of oxygen from bottom waters Coastal research in recent decades has identified key differences in the responses of lakes and coastal-estuarine ecosystems to nutrient enrichment The contemporary (Phase II) conceptual model reflects those differences and includes explicit recognition of (1) system-specific attributes that act as a filter to modulate the responses to enrichment (leading to large differences among estuarine-coastal systems in their sensitivity to nu- trient enrichment); and (2) a complex suite of direct and indirect responses including linked changes in: water transparency, distribution of vascular plants and biomass of macroalgae, sediment biogeochem- istry and nutrient cycling, nutrient ratios and their regulation of phytoplankton community composition, frequency of toxic/harmful algal blooms, habitat quality for metazoans, reproduction/growth/survival of pelagic and benthic invertebrates, and subtle changes such as shifts in the seasonality of ecosystem functions Each aspect of the Phase II model is illustrated here with examples from coastal ecosystems around the world In the last section of this review I present one vision of the next (Phase III) stage in the evolution of our conceptual model, organized around 5 questions that will guide coastal science in the early 21st century: (1) How do system-specific attributes constrain or amplify the responses of coastal ecosystems to nutrient enrichment? (2) How does nutrient enrichment interact with other stressors (toxic contaminants, fishing harvest, aquaculture, nonindigenous species, habitat loss, climate change, hydro- logic manipulations) to change coastal ecosystems? (3) How are responses to multiple stressors linked? (4) How does human-induced change in the coastal zone impact the Earth system as habitat for humanity and other species? (5) How can a deeper scientific understanding of the coastal eutrophication problem be applied to develop tools for building strategies at ecosystem restoration or rehabilitation?

https://www.int-res.com/articles/meps/210/m210p223.pdf

Our evolving conceptual model of the coastal eutrophication problem

Although algal blooms, including those considered toxic or harmful, can be natural phenomena, the nature of the global problem of harmful algal blooms (HABs) has expanded both in extent and its public perception over the last several decades. Of concern, especially for resource managers, is the potential relationship between HABs and the accelerated eutrophication of coastal waters from human activities. We address current insights into the relationships between HABs and eutrophication, focusing on sources of nutrients, known effects of nutrient loading and reduction, new understanding of pathways of nutrient acquisition among HAB species, and relationships between nutrients and toxic algae. Through specific, regional, and global examples of these various relationships, we offer both an assessment of the state of understanding, and the uncertainties that require future research efforts. The sources of nutrients poten- tially stimulating algal blooms include sewage, atmospheric deposition, groundwater flow, as well as agricultural and aquaculture runoff and discharge. On a global basis, strong correlations have been demonstrated between total phos- phorus inputs and phytoplankton production in freshwaters, and between total nitrogen input and phytoplankton pro- duction in estuarine and marine waters. There are also numerous examples in geographic regions ranging from the largest and second largest U.S. mainland estuaries (Chesapeake Bay and the Albemarle-Pamlico Estuarine System), to the Inland Sea of Japan, the Black Sea, and Chinese coastal waters, where increases in nutrient loading have been linked with the development of large biomass blooms, leading to anoxia and even toxic or harmful impacts on fisheries re- sources, ecosystems, and human health or recreation. Many of these regions have witnessed reductions in phytoplankton biomass (as chlorophyll a) or HAB incidence when nutrient controls were put in place. Shifts in species composition have often been attributed to changes in nutrient supply ratios, primarily N:P or N:Si. Recently this concept has been extended to include organic forms of nutrients, and an elevation in the ratio of dissolved organic carbon to dissolved organic nitrogen (DOC:DON) has been observed during several recent blooms. The physiological strategies by which different groups of species acquire their nutrients have become better understood, and alternate modes of nutrition such as heterotrophy and mixotrophy are now recognized as common among HAB species. Despite our increased un- derstanding of the pathways by which nutrients are delivered to ecosystems and the pathways by which they are assimilated differentially by different groups of species, the relationships between nutrient delivery and the development of blooms and their potential toxicity or harmfulness remain poorly understood. Many factors such as algal species presence/ abundance, degree of flushing or water exchange, weather conditions, and presence and abundance of grazers contribute to the success of a given species at a given point in time. Similar nutrient loads do not have the same impact in different environments or in the same environment at different points in time. Eutrophication is one of several mechanisms by which harmful algae appear to be increasing in extent and duration in many locations. Although important, it is not the only explanation for blooms or toxic outbreaks. Nutrient enrichment has been strongly linked to stimulation of some harmful species, but for others it has not been an apparent contributing factor. The overall effect of nutrient over- enrichment on harmful algal species is clearly species specific.

/pdf/harmful-algal-blooms-and-eutrophication-nutrient-sources-4ujytujxjq.pdf

Harmful Algal Blooms and Eutrophication: Nutrient Sources, Composition, and Consequences

Phytoplankton can become limited by the availability of nutrients when light and temperature are adequate and loss rates are not excessive. The current paradigms for nutrient limitations in freshwater, estuarine, and marine environments are quite different. A review of the experimental and observational data used to infer P or N limitation of phytoplankton growth indicates that P limitation in freshwater environments can be demonstrated rigorously at several hierarchical levels of system complexity, from algal cultures to whole lakes. A similarly rigorous demonstration of N limitation has not been achieved for marine waters. Therefore, we conclude that the extent and severity of N limitation in the marine environment remain an open question. Culture studies have established that internal cellular concentrations of nutrients determine phytoplankton growth rates, and these studies have shown that it is often difficult to relate growth rates to external concentrations, especially in natural situations. This should lead to a greater reliance on the composition of particulate matter and biomass-based physiological rates to infer nutrient limitation. Such measurements have demonstrated their utility in a wide variety of freshwater and marine environments, and, most importantly, they can be applied to systems that are difficult to manipulate experimentally or budget accurately. Dissolved nutrient concentrations are most useful in determining nutrient loading rates of aquatic ecosystems. The relative proportions of nutrients supplied to phytoplankton can be a strong selective force shaping phytoplankton communities and affecting the biomass yield per unit of limiting nutrient. A current dogma of aquatic science is that marine and estuarine phytoplankton tend to be nitrogen limited, while freshwater phytoplankton tend to be phosphorus limited. Carpenter and Capone (1983) documented the preeminence of N studies in the literature on brackish and marine ecosystems. In 1970 there were equal numbers of references per year to N and P. The decade of the seventies saw a nearly fourfold increase in references to N, while the number of P references per year remained essentially unchanged. No trend was evident for the freshwater literature despite the fact that by the late seventies the evidence for P limitation had become so great that phosphorus control was recommended as the legislated basis for controlling eutrophication in North American and European inland waters (e.g.

/pdf/nutrient-limitation-of-phytoplankton-in-freshwater-and-1gyy8avtuu.pdf

Nutrient limitation of phytoplankton in freshwater and marine environments: A review of recent evidence on the effects of enrichment1

Several hypotheses are advanced for resource relationships among planktonic diatoms in African freshwater lakes that are consistent with the light and nutrient conditions of the lakes and the extant and fossil distributions of the diatom species in them. The hypotheses are all testable and are potentially powerful tools for interpreting past climatic conditions. A ranking is proposed along a Si : P gradient: at the high end are the planktonic Synedra spp. with the highest Si requirements and lowest P requirements (high Si: P), the planktonic Nitzschia spp. are intermediate, and the Stephanodiscus spp. are at the low end with the lowest Si requirements and highest P requirements (low Si : P). Melosira species may be ranked along a light: P gradient. We suggest that Melosira distans and Melosira arnbigua grow under high light and have low P requirements, Melosira agassizii and Melosira granulata are intermediate, and Melosira nyassensis has the lowest light and highest P requirements. There also appears to be a relationship between pore size and the light regime for growth among the Melosira species; thus, M. distans and M. ambigua have the smallest pores and highest light requirements, M. nyassensis has the largest pores and lowest light requirements. Melosira granulata is intermediate and seems to be very variable in pore size, depending on the light environment. One diatom, Nitzschia fonticola, lives in and on colonies of Macrocystis and is considered to be an obligate nitrogen heterotroph.

/pdf/hypothesized-resource-relationships-among-african-planktonic-1n64a52b6q.pdf

Hypothesized resource relationships among African planktonic diatoms

Sinking rates of Asterionellu formosa, Melosira agassixii, Cyclotella meneghiniana, and Scenedesmus quadricauda in stationary and exponential phases of growth arc rcportcd. Stationary phase populations sink 4~ more rapidly, on the average, than exponentially growing populations. Time series of sinking rates during change from one growth phase to another demonstrate the viability of rapidly sinking cells. A consideration of sinking in Langmuir circulations indicates that the frcqucntly used algebraic relationship between sinking rate and rate of loss of cells from the mixed layer may greatly ovcrcstimatc loss rates. A theoretical net potential growth curve, that combines both loss from sinking and growth from sinking-dcpcndent nutrient uptake, demonstrates that nutrient depleted cells may have optimal growth rates (highest fitness) at high sinking rates.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1976.21.3.0409

Sinking in freshwater phytoplankton: Some ecological implications of cell nutrient status and physical mixing processes1

Examination of models of nutrient-limited growth and uptake, nutrient patchiness, resource competition, and rand K-selection with respect to marine and freshwater phytoplankton indicates that organisms from both habitats are ecologically similar. However, there are differences in the physiology and ecology of phytoplankton within a given habitat (either marine or freshwater) during the course of succession. Even though the identity and basic ecology of organisms that are present early and late in successional sequences are generally known, little is known about their physiology. Marked differences in uptake ability, storage capacity, and growth and loss rates will be found for phytoplankton that can be ranked along an rthrough K-selection continuum. This continuum can serve as a unifying concept in phytoplankton ecology. There are numerous similarities between marine and freshwater phytoplankton, but papers and reviews that deal with their ecology often restrict coverage to organisms from one habitat or the other. In this paper we will examine a variety of topics currently of interest to phytoplankton ecologists in order to determine similarities or differences between phytoplankton from each environment. These topics include models of nutrient-limited uptake and growth, nutrient patchiness, resource competition, and rand K-selection. Several books on the ecology of marine (Morris 1980; Platt 198 1; Carpenter and Capone 1983) and freshwater (Reynolds 1984) phytoplankton have appeared recently, and they should be consulted for detailed information. In this review we will also contrast and compare the ecology and physiology of marine and freshwater phytoplankton that can be found along gradients of resource availability (e.g. varying nutrient levels) in either environment. There are substantial differences in the proportions of phytoplankton species representing the major divisions of algae in marine and freshwater environments, and sev’ An international travel award from the National Science Foundation supported travel to the Symposium on the Comparative Ecology of Freshwater and Coastal Marine Ecosystems in Nairobi, Kenya. era1 important groups (classes and orders) are representative of one habitat or the other. The marine phytoplankton is dominated by numerous species of Chrysophyta (diatoms, coccolithophores, and silicoflagellates: Bold and Wynne 1985) and Pyrrhophyta (dinoflagellates). Several other groups of algae are at times either conspicuous or abundant, but they are represented by very few species. These include the Cyanophyta [=cyanobacteria; e.g. very small-celled species of Synechococcus or large bundles of Oscillatoria (Trichodesmium) filaments]. Another characteristic member of the marine phytoplankton is Halosphaera (division Chlorophyta, class Prasinophyceae), which has spherical green cells. Freshwater phytoplankton is well represented by species of most of the major divisions of algae that have a planktonic component. Therefore, a plankton sample from a lake may contain Cyanophyta, Chlorophyta, Chrysophyta, and Pyrrhophyta. Euglenophyta, which can usually be found in small ponds, are not common members of the freshwater phytoplankton (Round 198 1). A large number of species of Chlorophyta and Cyanophyta are found in freshwater, but not in seawater. Coccolithophores, which are characteristic of marine plankton, are rarely observed in freshwaters. Marine and freshwater ecologists generally approach the study of phytoplankton

/pdf/comparative-ecology-of-marine-and-freshwater-phytoplankton-segljt0tz5.pdf

Comparative ecology of marine and freshwater phytoplankton

SUMMARY. 1. Fossi! diatom assemblages deposited in more than a dozen African lakes roughly y5(X) years BP were dominated by a single planktonic species, Stephanodiscus astraea (Ehrenb.) Grun. (although realistically this is likely to be a species complex). These diatoms flourished when lake-levels were maximal. Data are included from many of the large African lakes, and others extending from Lake Abh6, Ethiopia, to Lake Cheshi. Zambia. 2. Because the ecological physiology of Stephanodisctts species is well known one can predict the nutrient regime that must have existed when Stephanodiscus bloomed. Owing to competition for resources Stephanodiscus species dominate when the supply ratio of silicon to phosphorus (in moles) in the epilimnion is relatively low (Si:P—1). Consequently, lakes dominated by S. astraea are often hypereutrophic. 3. We propose a series of hypotheses to explain why tropical lakes have decreasing Si:P ratios as lake-levels increase, primarily owing to internal P-loading processes in the epilimnia. These observations appear to contradict present conceptions of the fundamental relationships governing nutrient loadings to and within lakes. Tropical lakes appear to have had increasing epilimnetic phosphorus loading as lake-levels increased. In contrast, large, deep lakes in the temperate zone are usually oligotrophic, with high Si:P ratios. 4. Our major conclusion is that regeneration rates are greater than removal rates for phosphorus in tropical lakes as compared to temperate lakes, especially where epilimnetic mixing exceeds 50 m. Biological control of the elemental cycles dominate in tropical lakes, whereas nutrient cycles in temperate lakes are dominated by physical processes for a large part of the year. This results in major differences in the fundamental mechanisms of nutrient regeneration and their relationships to morphometric features of lakes in the two regions.

/pdf/opinion-endless-summer-internal-loading-processes-dominate-1nh1hk9qmd.pdf

Peter Kilham

Papers

Nutrient limitation of phytoplankton in freshwater and marine environments: A review of recent evidence on the effects of enrichment1

Hypothesized resource relationships among African planktonic diatoms

Sinking in freshwater phytoplankton: Some ecological implications of cell nutrient status and physical mixing processes1

Comparative ecology of marine and freshwater phytoplankton

OPINION Endless summer: internal loading processes dominate nutrient cycling in tropical lakes