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?

Microalgal biomarkers: A review of recent research developments

The question of nutrient limitation of primary production in estuaries and other marine ecosystems has engendered a great deal of debate. Although nitrogen is often named as the primary limiting nutrient in seawater (3, 17-19, 50, 52, 55, 61, 76, 80), this is by no means universally accepted. Many workers have argued that phosphorus is limiting (58, 71), that both nitrogen and phosphorus can simultaneously be limiting (9), or that primary production can switch seasonally from being nitrogen-limited to phosphorus-limited (6, 46). Others argue that nutrients are not limiting at all in many marine ecosystems, including highly oligotrophic waters (15). To some extent these disagreements result from poor communication due to different definitions of nutrient limitation. Considerable argument also occurs over the various methods and approaches used to estimate nutrient limitation. Limnologists in particular have tended to be critical of the methods often used to study nutrient limitation in marine ecosystems (23). Nutrient limitation in lakes has historically received more study than that in estuaries, and most mesotrophic and eutrophic north-temperate lakes are phosphorus limited (8, 62, 63, 66, 81). Thus, there is a feeling among many limnologists and environmental engineers who study lakes that marine ecosystems also probably are phosphorus limited. Lacking strong mechanistic arguments to explain why nutrient limitation might be different in estuaries than in lakes, environmental management agencies often assume that phosphorus limitation in marine ecosystems is the rule.

Nutrient Limitation of Net Primary Production in Marine Ecosystems

Rapid mass sinking of cells following diatom blooms, observed in lakes and the sea, is argued here to represent the transition from a growing to a resting stage in the life histories of these algae. Mass sinking is of survival value in those bloom diatoms that retain viability over long periods in cold, dark water but not in warm, nutrient-depleted surface water. Mechanisms for accelerating sinking speed of populations entering a resting or “seeding” mode are proposed. Previously unexplained features of diatom form and behaviour take on a new meaning in this context of diatom seeding strategies. Diatoms have physiological control over buoyancy as declining growth is accompanied by increasing sinking rates, where the frustule acts as ballast. Increased mucous secretion in conjunction with the cell protuberances characteristic of bloom diatoms leads to entanglement and aggregate formation during sinking; the “sticky” aggregates scavenge mineral and other particles during descent which further accelerates the sinking rate. Such diatom flocs will have sinking rates of ∼ 100 m d-1 or more. This is corroborated by recent observations of mass phytoplankton sedimentation to the deep sea. This mechanism would explain the origin of marine snow flocs containing diatoms in high productivity areas and also the well-known presence of a viable deep sea flora. That mortality is high in such a seeding strategy is not surprising. A number of species-specific variables pertaining to size, morphology, physiology, spore formation and frustule dissolution rate will determine the sinking behaviour and thus control positioning of resting stages in the water column or on the bottom. It is argued that sinking behaviour patterns will be environmentally selected and that some baffling aspects of diatom form and distribution can be explained in this light. Rapid diatom sedimentation is currently believed to be mediated by zooplankton faecal pellets, particularly those of copepods. This view is not supported by recently published observations. I speculate that copepod grazing actually retards rather than accelerates vertical flux, because faecal pellets tend to be recycled within the surface layer by the common herbivorous copepods. Egestion of undigested food by copepods during blooms acts as a storage mechanism, as ungrazed cells are likely to initiate mass precipitation and depletion of the surface layer in essential elements. Unique features of diatoms are discussed in the light of their possible evolution from resting spores of other algae. An evolutionary ecology of pelagic bloom diatoms is deduced from behavioural and morphological characteristics of meroplanktonic and tychopelagic forms. Other shell-bearing protistan plankters share common features with diatoms. Similar life-history patterns are likely to be present in species from all these groups. The geological significance of mass diatom sinking in rapidly affecting transfer of biogenic and mineral particles to the sea floor is pointed out.

Role of sinking in diatom life-history cycles: ecological, evolutionary and geological significance

Role of large particles in the transport of elements and organic compounds through the oceanic water column

SHALLOW near-shore ocean waters support high primary production because of the availability of inorganic nutrients. The availability is usually attributed to the proximity of fresh-water runoff or to coastal upwelling and deep water advection1,2.

Benthic nutrient regeneration and its coupling to primary productivity in coastal waters

In response to Thiel's (1975). hypothesis that the food-limited deep sea is a small organism habitat, further data on average size of individuals representing various deep-sea taxa are presented. Our data were gathered with trawls and box corers between 200 and 5000 meters, in the western North Atlantic. For echinoderms, decapods and macrofauna there appears to be no steady, logarithmic decline in size with increasing depth, but fishes are bigger – deeper.

The Size‐Depth Relationship in Deep Ocean Animals

Suspended and sinking particulate matter from the water column and sediment and fecal pellets from Unit I sediments were collected in the central basin of the Black Sea for organic geochemical investigations. The hydrocarbon compositions of these samples proved informative for providing information on the sources for the particulate and sedimentary organic matter. Hydrocarbons derived from planktonic, terrestrial, bacterial, and fossil sources can be distinguished. Plankton-derived hydrocarbons dominated the 53 µm particles. By contrast, 53 µm) particles in the anoxic zone contained equal amounts of hydrocarbons from all four sources. Fecal pellets and sediments contained the greatest contribution of hydrocarbons of terrestrial origin (up to 56% of total hydrocarbons).

Organic Matter Sources in the Black Sea as Inferred from Hydrocarbon Distributions

The relationship between lipid composition and seasonal differences in the distribution of PCBs in Mytilus edulis L.

Anchoveta fecal material links production and sedimentation of particulate matter, including organic carbon, organic nitrogen and diatom frustules, in the Peruvian coastal upwelling ecosystem. Longterm fluctuations in anchoveta stock could, therefore, affect the quantity and composition of particulate matter input to the benthos and thereby to the sediment record.

C. Hovey Clifford

Papers

Benthic nutrient regeneration and its coupling to primary productivity in coastal waters

The Size‐Depth Relationship in Deep Ocean Animals

Organic Matter Sources in the Black Sea as Inferred from Hydrocarbon Distributions

The relationship between lipid composition and seasonal differences in the distribution of PCBs in Mytilus edulis L.

Downward Transport of Particulate Matter in the Peru Coastal Upwelling: Role of the Anchoveta, Engraulis Ringens