Published data and analyses from temperate and tropical aquatic systems are used to summarize knowledge about the potential impact of land-use alteration on the nitrogen biogeochemistry of tropical aquatic ecosystems, identify important patterns and recommend key needs for research. The tropical N-cycle is traced from pre-disturbance conditions through the phases of disturbance, highlighting major differences between tropical and temperate systems that might influence development strategies in the tropics. Analyses suggest that tropical freshwaters are more frequently N-limited than temperate zones, while tropical marine systems may show more frequent P limitation. These analyses indicate that disturbances to pristine tropical lands will lead to greatly increased primary production in freshwaters and large changes in tropical freshwater communities. Increased freshwater nutrient flux will also lead to an expansion of the high production, N- and light-limited zones around river deltas, a switch from P- to N-limitation in calcareous marine systems, with large changes in the community composition of fragile mangrove and reef systems. Key information gaps are highlighted, including data on mechanisms of nutrient transport and atmospheric deposition in the tropics, nutrient and material retention capacities of tropical impoundments, and N/P coupling and stoichiometric impacts of nutrient supplies on tropical aquatic communities. The current base of biogeochemical data suggests that alterations in the N-cycle will have greater impacts on tropical aquatic ecosystems than those already observed in the temperate zone.

/pdf/the-impact-of-accelerating-land-use-change-on-the-n-cycle-of-23xlceybhm.pdf

The impact of accelerating land-use change on the N-Cycle of tropical aquatic ecosystems: Current conditions and projected changes

Nutrient limitation of primary production was experimentally assessed using an in situ bioassay technique in the Quebrada Salto, a third-order tropical stream draining the northern foothills of the Cordillera Centrall in Costa Rica . Bioassays employed artificial substrata enriched with nutrients that slowly diffuse through an agar-sand matrix (Pringle & Bowers, 1984) . Multiple comparisons of regression coefficients, describing chlorophyll-a accrual through time for different nutrient treatments, revealed positive micronutrient effect(s) . Micronutrient treatment combinations (Fe, B, Mn, Zn, Co, Mo, EDTA), supplemented with and without nitrate and phosphate, exhibited significantly greater chlorophyll-a accrual over all other treatments (P < 0.05), supporting over three times that of the control after 14-d of substratum colonization . Neither of the major nutrients (N or P) produced a significant stimulation, although the N treatment displayed =50% more chlorophyll-a than the control after 14-d . Similarly, Si, EDTA, and Si + N + P treatments did not exhibit chlorophyll-a response curves that were significantly different from the control . During the experiment, mean NH 4-N and (NO2+ NO 3)-N concentrations in the Salto were 2 .0 µM (28 .6 µg•1-')and 7.2 µM (100.2 µg •1-'),respectively . High concentrations of P0 4-P (x =2.0 µM; 60.9 µg•1-')and TP (x =3.0 µM; 94.0 µg•1-1)were also found, and consequently low molar N

/pdf/in-situ-nutrient-assays-of-periphyton-growth-in-a-lowland-2x5fgp4tfb.pdf

In situ nutrient assays of periphyton growth in a lowland Costa Rican stream

SUMMARY In 1981, the UNESCO in association with the IIASA and OECD recognized that tropical/-sub-tropical water bodies may respond differently to eutrophication management techniques than heretofore expected on the basis of temperate lake studies However, little information on the eutrophication process and eutrophication management appeared to be available in the literature This paper reports on the results of a survey of the published literature dealing with eutrophication and eutrophication management in tropical/sub-tropical regions, and highlights the modifying effects of seasonality, climate and hydrology on the eutrophication process in these regions Nevertheless, the causes and correctives of eutrophication do not differ except in degree from those in temperate regions

Aspects of eutrophication management in tropical/sub-tropical regions

Low (0.1–0.7 µmol liter−1) dissolved inorganic nitrogen concentrations are common in the floodplain lakes of the Parana River, where the water hyacinth (Eichhornia crassipes) is the major primary producer. We tested the hypothesis that N supply could limit production of E. crassipes during the growth season by adding NH4+ or PO43− during 1 month to large in situ enclosures. Those enclosures amended with N produced twice as much biomass as the controls and P-treated enclosures. The low and limiting N content (1.3% AFDW) of the control and P-treated plants provide a first diagnostic value for N deficiency in the field for E. crassipes. N limitation in these lakes may be attributable to high P supply from Andean rivers, high losses of macrophyte N during floods, and the unstable nature of floodplain lakes.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.1994.39.2.0439

Limitation of water hyacinth by nitrogen in subtropical lakes of the Paraná floodplain (Argentina)

A series of water samples from Lake Kinneret was supplemented with 100 µM N (as NH4 or as NO3 and/or 10 µM orthophosphate-P. The yield of phytoplankton both as chlorophyll and in cell numbers of major species was determined after a two-week incubation. During these experiments, some of the algae present initially never multiplied (e.g. Peridinium and Peridiniopsis spp. Cryptomonas spp., Rhodomonas spp. and Crysochromulinaparva); others e.g. Anomoeoneis exilis, Synedra sp., Chlamydomonas sp., Elakatothrix gelatinosa), undetected in the original sample, grew out during the incubation. Chlorophyte species (the majority of commonly observed forms in the lake) responded most readily to added nutrients.

Lake Kinneret phytoplankton: Response to N and P enrichments in experiments and in nature

The effects of enrichment with phosphate (0–500 µg. 1−1) and forms of nitrogen (nitrate, nitrite, ammonia an and urea) (0–3500 µgg. −1) on the phytoplankton growth of Lobo Reservoir (Brazil) were studied in July, 1979. Suspended matter, chlorophyll a, cell concentrations and the carotenoid:cchlorophyll ratio were estimated following 14 days of ‘in situ’ incubation. Phosphate alone caused no significant effects, but enrichment with nitrogen caused a substantial increase on the growth of phytoplankton. Comparison between the different forms of nitrogen showed insignificant effects after their additions with 350 µg. −1 and in combination with phosphate. However, when nitrogen was added in large quantities (3 500 µg. −1), significant differences between the nitrogeneous forms were found, with urea causing the strongest effect. In July, nitrogen is mhe main limiting nutrient to phytoplankton growth of Lobo Reservoir.

P. R. Curi

Papers

Effects of phosphorus and nitrogen enrichment on the phytoplankton in a tropical reservoir (lobo reservoir, brazil)